Pracatical Stuff Part-2

 

Practical HART — Part 2

 

Part 2:  Practical Stuff

A Caveat:  HART and Current Consumption

    Adding HART to an analog 2-wire transmitter eats into the available current in two ways.  First, there is the current consumed by HART functions.  And, second, there is less current to start with because of the superposition of the HART signal.  If the analog output is 4 mA, then the instantaneous output during HART transmission can typically drop to 3.5 mA.   This often means that there is only 3.5 mA available to power circuitry.   Alarm conditions and guard bands can further erode this number, as illustrated in figure 2.1.   Energy storage methods can prevent the loss of 0.5 mA, but might be unsatisfactory in an intrinsically safe device.

wpe35.gif (5419 bytes)

Figure 2.1 — Available Operating Current With HART


Modem Sources

    When people talk about modems, it’s not always clear whether they mean an integrated circuit that can be designed into their product or a completed, network-ready unit (HART Master).  For information on HART modems of either type, see    Romilly Bowden.    The HART Communication Foundation is another source of information.  For just the integrated circuits, you might also want to check out our paper entitled   HART Chips:  Past, Present, Future.

 

HART Library Software For PC

    HART Device Drivers are available from   Borst Automation   .  This allows you to put buttons, etc. on your screen that read and write HART parameters, put them into spreadsheets, etc.  Also, see the section entitled   HART and PCs   .

 

HART and PCs

    The combination of a Personal Computer and Serial Port HART Modem is often used as a HART Master.  In the days of DOS this was easier because you could write software that would take over the whole computer and generate the proper timing.  Nowadays it isn’t so easy.  The very enhancements, namely Windows and buffered UARTs, that make PCs more useful for other applications have made them less useful for HART.  Windows 95 and 98 have no provision for real time activities, and delays of 20 to 100’s of milliseconds are reported [2.1].  Windows NT has provision for “Real-time Threads”.  But experiment shows [2.2] that it can still devote 100% of CPU time to a task and ignore I/O events completely.  Since one character time in HART is 9.2 millisecond, the delays involved can be several character times.  This is enough to destroy HART Master arbitration.  So-called RTOS extensions to Windows NT are available, that can make Windows NT appear to be more of a real-time operating system.  But this is extra software to buy, install, and understand.  Worse yet, the HART application software may depend on whose RTOS is being used, so that it becomes tied to the RTOS instead of the PC.

    In some applications, where it is known that there will be only one HART Master and where HART burst-mode is not used, a Windows-based PC and simple Modem can still be used.  The only timing consideration is how long to wait for a Slave to reply.  Such applications don’t follow HART Specifications and don’t allow Master arbitration.  Nevertheless, they are useful and probably represent a fairly large subset of HART software.  You can download source code (in C) that works in this manner and does a lot of general HART activities, such as extracting device information, calibration, etc.  It runs in a DOS window under Windows.  Click   here   to download.

    To address the real-time requirements of HART, some systems put another processor between the PC and the Modem.  This can take the form of either a single-board-computer or an embedded microcontroller that is part of the Modem.  The single-board-computer or embedded controller forms a buffer between the Modem and the PC and takes care of all of the HART timing.

    A recently introduced integrated circuit, the “P51”, from Cybernetic Micro Systems, Inc. addresses the timing problems of PCs.  It appears to have almost everything you need to make a full HART modem that plugs into a PC’s ISA bus.   It is based on an 8051 and contains a complete interface to a PC or PC104 bus, including dual-port RAM and interrupts.  In this case the 8051 does the HART communication and provides the timing.

    Even a newer computer running DOS won’t always perform as expected, because of the way that the UARTs work.  Most modern UARTs in PCs are usually equipped with built-in FIFOs, to avoid frequent interrupts of the CPU.  This is a swell idea, except that the UART doesn’t put error information into a FIFO.  If there is an error, such as a parity error, there is no way of knowing which byte in the FIFO had the parity error (or whether more than one byte was in error).  Consequently, there is no way of weeding out the initial HART preamble bytes that are in error because of carrier start-up.  (See the section entitled Start-Up Synchronization in HART for details.)  Fortunately, the UART can often be programmed so that the FIFO is disabled, allowing you to associate error status with each data byte.

    Commercially available software packages and libraries for data communication are another source of trouble for the would-be HART Master.  Most of them are geared toward telecom modems and have no concept of burst modems.  They invariably turn on RTS (request-to-send) and assume it should be ON forever.  (HART Modems require RTS to be on only during transmit.)  They also are good at losing error status, just like FIFOed UARTs.  They let you set up a receive buffer, for example.  But they don’t let you set up a corresponding buffer of error status.   You receive a notice telling you there’s a HART message in the buffer, and another notice saying that some of the bytes are in error.  But you don’t get to know which ones are in error.  Finally, one other nasty thing the software package will do is to make sure that your UART FIFO is turned ON.

    Another UART caveat is that if you read a PC-based UART status, the status is automatically cleared.  If you need to use the status word more than once, make sure that you store it after the first read.

Timing is Everything

    HART allows two Masters.  Arbitration is used to determine which one will use the network.  The arbitration is based on monitoring of network traffic and implementation of timers.  A Master that is aware of what has recently transpired is said to be synchronized.  An unsynchronized Master is one that has either lost synchronization or has recently been connected to the network and has yet to become synchronized.  Loss of synchronization occurs if the processor in the Master must temporarily stop monitoring network traffic to do other things, or if there is no network traffic, or if there are message errors that prevent it from knowing what’s happening.

    If two Masters are present and both are synchronized, then they will use the network alternately.  This assumes, of course, that both have something to say.  If one of them doesn’t it can give up its turn but still remain synchronized.   This is illustrated in figure 2.2.  The Slave Response in each case may be from a different Slave.

wpe38.gif (9509 bytes)

Figure 2.2 — Master Alternation

During this process a given Master knows that it is free to use the network when it sees the end of the Slave response to the other Master.  If a Master doesn’t take its turn, the other Master can have another turn, provided it waits a length of time called RT2.  The time interval RT2 is illustrated in figure 2.2.  The Master whose turn it is to use the network has this much time in which to start.  Otherwise the Master that last used the network may start.  This is how things role merrily along when there are no problems and when both Masters have almost continuous business to transact.   Although not explicitly shown in figure 2.2 and subsequent figures, both Masters start their timers at the end of any network activity.  Any fresh activity cancels the timers.  Also, it is implicit in these explanations that a Master will not begin talking if someone else is talking.

    Now suppose that, as a result of a message error, a Slave doesn’t respond to Master 1.  Master 2 must now wait a length of time called RT1 before it tries to use the network.  Master 1, while waiting for the Slave response, sees the Master 2 command instead.  It then waits until Master 2 is done and then it can retry.  This is illustrated in figure 2.3.

wpe39.gif (4991 bytes)

Figure 2.3 — Master Alternation with No Slave Response to Master 1

Here, Master 2 has lost synchronization because it did not see a Slave Response to Master 1.  It regains synchronization at the end of RT1.

    Suppose, in figure 2.3, that the Slave finally did respond to the Master 1 command before the end of RT1.  Then things would have proceeded normally.   RT1, which is longer than RT2, is approximately the length of time that a Slave is allowed to respond.  Actually, the Slave maximum response time, which is designated TT0, is slightly shorter than RT1.  This ensures that a Master and Slave will not start transmitting simultaneously.

    If a Master is new to the network, then it must wait a length of time RT1 before it tries to use the network.  At the end of RT1 it has become synchronized and may use the network.  Or else, if it sees and recognizes a transaction of the other Master before RT1, then it is immediately synchronized.

    In another scenario suppose that a Slave has responded to Master 1, but the response appeared garbled to Master 2.  Figure 2.4 shows what happens.

wpe3A.gif (4909 bytes)

Figure 2.4 — Alternating Masters with Master 2 Failing to Recognize Slave Response to Master 1

Since Master 2 didn’t see a good Slave Response, it begins waiting a length of time RT1 from the end of the Slave Response.  Master 1, which saw a good Slave Response and is still synchronized, starts RT2.  At the end of RT2, Master 1 sees that Master 2 isn’t using the network and decides to use it again.  Master 2 sees this new transmission by Master 1 and becomes resynchronized.  Had Master 1 not wanted to re-use the network again, then Master 2 would have become resynchronized at the end of RT1 and could have begun its transaction then.

    If neither of the Masters needs to talk, the two Masters become unsynchronized.  In effect, either Master knows it has waited a time RT1 and can begin again whenever it needs to.

    Suppose that both Masters are new to the network or are both unsynchronized and try to use the network at the same time (after waiting for RT1).  The respective commands will be garbled and there will be no response.  Both Masters will start RT1 again at about the same time.  And both will collide again at the end of RT1.  To prevent this from going on endlessly, the Primary and Secondary Masters have different values for RT1.  The Primary Master uses a value designated RT1(0).   The Secondary Master uses a value designated RT1(1).

    How do things work if there is a Slave in burst-mode?   Arbitration is simple if there is a Slave in burst-mode.  To see this, recall that the bursting Slave will be the only Slave on the network.  Following each burst it must wait a short time to allow a Master to use the network.  The Protocol requires that the bursting Slave alternate information in its bursts, making it appear that alternate bursts are Responses to alternate Masters.  Masters watching the network will see a burst that is a Response to Master 1, followed by a burst that is a Response to Master 2, followed by a burst that is a Response to Master 1, and so on.   A given Master knows it can use the network following a burst that is a Response to its opposite.  That is, if a given burst was a response to Master 2, then Master 1 knows that it may use the network at completion of the burst.  In this strange turn of events, the Slave gets to decide who is next.

    Values of the timers are given in table 2.1.

Timer Description Symbol Value (character times)
Master Wait Before Re-Using Network RT2 8
Primary Master Wait from Unsynched RT1(0) 33
Secondary Master Wait from Unsynched RT1(1) 41
Slave Max time to Respond TT0 28
Slave Time Between Bursts BT 8

Table 2.1 — Timer Values

TT0, the length of time in which a Slave must respond, is deliberately made quite large to accommodate less capable hardware and software that is likely to be found in a Slave.  RT1(0), in turn, has to be larger than TT0.  And RT1(1) has to be larger than RT1(0).    The various timer values have been carefully set to account for various hardware and software latencies.   It would probably have been possible to omit RT2 and just force Masters to resynchronize (using RT1) after every Master or Slave Response.  However, since RT2 is much smaller than RT1, the existence of an RT2 allows much faster arbitration.

 


The Beginning, End, Gaps, and Dribbles

    The previous section on arbitration shows the importance to a Master of knowing when a message ends.  In fact, both Masters and Slaves need to be aware of when a message starts, stops, or is present.  This is not entirely straightforward, and depends on a combination of (1) carrier detect, (2) UART status indications, and (3) monitoring message content.

    Carrier detect indicates that a carrier of acceptable amplitude is present.  It tells a device that it should be examining its UART output and UART status.  In the UART status, a “receive buffer full” (RBF) indication will occur once each character.  Whether a message is present is determined by the combination of carrier detect and the RBF indications.  Many devices don’t directly monitor carrier detect.  Instead, they use it to qualify (gate) the UART input.  This bypasses the additional step of having to look at an I/O line.

    The presence of RBFs indicate that a message is present.  But they don’t necessarily indicate the end of a message or the start of another.  Back-to-back messages can occur (see box below), which means that a new message starts simultaneously with the end of a previous message.   The transition from one message to another can only be detected by monitoring message content.  The start of a message is indicated by a 3-character start delimiter.  This delimiter is a sequence that isn’t likely to occur anywhere else in a message.  It is more completely described in the section entitled   Start-Up Synchronization in HART.   A device will normally be looking for this start delimiter sequence unless it has already seen the sequence and is simply parsing the rest of the message as it arrives.

    But, what if the start sequence does appear in “normal data”?  This is a weakness of HART, but probably not a very important one.   The reason is that Slaves are probably the only devices that do not fully parse each message.  Therefore, if a start sequence occurs in mid-message, only a Slave will be fooled into thinking that it sees the start of the next message.   This Slave will then look for its own address, a command, etc.  The chance that the rest of the byte sequence will contain the Slave’s 38 bit address is probably almost non-existent.  Therefore, the Slave will not see its own address and will resume the normal activity of looking for the start sequence.

Back-to-back Messages and Temporary Collisions:    A device will often parse the entire message and know, upon receipt of the checksum, where the message ends.  A Slave may do this, for example, if the message was addressed to it.  Masters do it as part of   arbitration.  The Slave that is supposed to respond may immediately assert its own carrier upon seeing the checksum.  Similarly, the Master may realize that it will have the next use of the network and assert its own carrier upon seeing the checksum.   The new carrier may be asserted before the previous one has been removed and before the incoming RBFs stop, leading to a temporary collision.  During this time carrier detect never drops.     A temporary collision may sound like something terrible, but it has the same effect and is no more of a problem than carrier start-up alone.  Carrier start-up is more completely described in the section entitled  Start-Up Synchronization in HART.

    If a message should become garbled, then devices that have been parsing it must revert to waiting for the RBFs or carrier detect to stop, or for a new start sequence to appear.

    Ideally, RBFs would occur at a constant rate of one every 9.17 millisecond and the last one would correspond to the checksum character.   But received messages can have two peculiarities called gaps and dribbles.  A gap can occur between two characters of the same message.  It is a delay from the end of the stop bit of one character until the start bit of the next character.  It will appear to be an extension of the stop bit (logic high at UART input).  A dribble is an extra character that appears at the end of a message, just after the checksum character.  A dribble isn’t transmitted and doesn’t appear on the network.  It is manufactured by the receive circuit/demodulator/UART, possibly as a result of the carrier shutting OFF.  It will be shown here that these really don’t affect anything, except to slow down communication.

    Gaps occur when a Slave is not able to keep up with the 1200 bits/second data rate.  In theory there could be a gap between every two characters of the received message.  During a gap the carrier is present but no information is being sent.  Most modern Slaves are probably able to transmit without gaps.  But we still must assume that they can occur.  The HART specifications seek to limit gaps to insure maximum throughput, but are ambiguous as to how large a gap can be.  One bit time and one character time are both specified.  The ambiguity probably reflects the fact that a gap size on the order of 1 character time or less doesn’t matter much.  In the following we assume a maximum of one character time.

    Normally, RBFs occur at a rate of one per character time throughout the message.  If there is a gap, then there could be up to two character times between RBFs.  A device that is trying to decide whether a message has ended will normally restart its timer on each RBF.  The timer must be at least two character times (18.33 millisec) to account for the possible gap.  Masters will start RT1 and RT2 timers, both of which are much longer than a gap time.  Therefore, arbitration will not be affected by a gap.  A Slave that is being addressed may also implement a timer, so that it can detect truncated messages.  This timer must also be longer than two character times.

    A dribble generates an extra RBF.  It occurs so soon after a preceding character that it simply restarts timers and does not affect arbitration.  A device that creates this extra RBF will have to read and discard the phantom character.  And, since it will not be able to tell the difference between the phantom and a real transmitted character, it may have to check the character to see whether it is part of a start sequence.

    To summarize, the presence of a message is indicated by the combination of carrier detect and RBFs.  Since back-to-back messages can occur, it is not acceptable to look for carrier detect drop-out as an end of message.   Devices must look for the 3-character start sequence.  Gaps and dribbles can also occur in a received message.  Provided that device timers are longer than 2 character times, gaps and dribbles have no effect except to slow down communication.

 


Start-Up Synchronization in HART

    HART is a type of data communication in which devices assert carrier only for the time it takes to send a message.  (The modems are also called “burst modems.”)  When it’s time to talk, a device starts up its carrier and begins modulating it with the desired data.  When it is finished talking the device drops its carrier.   Devices that are listening must determine where the data starts.  If a listener fails to locate the starting point, then the message will generally appear meaningless.

    The chosen protocol must, therefore, provide some way for listeners to reliably locate the start of the data.  A common way to do this is to send an initial pattern of bits or symbols — a preamble or start delimiter — that is known to all listeners.  A challenge is always to make the preamble and/or start delimiter short to keep overhead low.  Another challenge arises because of the start and stop of the carrier.  There is generally no way to insure that this happens in a “clean” fashion.  Initial bits will appear to change randomly as the carrier rises to full amplitude, filter circuits settle, etc.  There may also be “dribble” bits at the end.

    The originators of HART faced this problem:  If carrier start-up causes random bits to be applied to the UART, how do we create an unambiguous start of data?     The UART uses a start bit (logic zero) to synchronize reception of one character.  If it’s been sitting idle for a time or if it thinks that the last thing it got was a stop bit, then the UART must assume that the next zero bit or the next transition to zero, is a start bit.  Any initial random zero bit will be considered a start bit.  Then, after 10 more bit times the UART will take the next zero bit as the next start bit.  Data containing a normal mix of ones and zeros will confuse the UART by presenting it with zeros that it thinks are start bits.

    The solution in HART is to start the message with a string of characters whose only zero bits are start bits.  The UART may be confused at first.  But after one or two characters it becomes synchronized.   There is only one character that is all ones.  It is formed by adding odd parity to 0xff.  The start-up process is illustrated in figure 2.5.

about2_5.gif (5744 bytes)

Figure 2.5 — Start-Up Synchronization

Here the modems have caused a delay between the transmit and received UART signals.   At carrier start-up there are some garbage bits at the receiving UART’s input.   This causes the UART to begin assembling a character.  When it has finished it will present this garbage character to the processor.  Then it will wait for the next start bit.  It won’t find one until after the “gap” has passed.  Then it will begin assembling the first good character.  The processor looks for a 0xff byte (good character) and discards the initial non-0xff bytes.

    The receiving processor looks for a sequence of 3 contiguous bytes:   preamble, preamble, start delimiter.  Thus, at least two good preamble characters must be received and they must be those that immediately precede the start delimiter.  HART requires that a minimum of 5 preamble characters be transmitted.   This allows for the loss of up to 3 characters by the process just described.   Typically 1 or 2 characters are lost.

    Repeaters typically cause a loss of preamble character because they must listen for carrier at both ends and then “turn the line around.”  The fact that there is only about one character to spare means that a HART repeater must do this in under one character time.  Usually this is enough time.

    Another possible way around the start-up problem would have been to have transmitting devices turn on their carrier and force it to 1200 Hz (logic 1) and wait for a few character times before loading the UART to begin data transmission.  If the transmitting UART is simply left empty before transmission the output will be 1200 Hz, equivalent to a stop bit or idle condition.  This is the same as creating a deliberate gap of a few character times.  At the receivers the respective UARTs would all collect an initial garbage character, as in figure 2.5.  But then there would be a gap, followed by the start bit of the first transmitted character.  This method has the drawback of requiring transmitting devices to implement a gap timer at the start of the message.

    A weakness of HART is that message start sequence of (preamble, preamble, start delimiter) can occur in data.   A device looking for a start sequence must look at context to determine whether these 3 characters represent a delimiter or data.  This makes HART somewhat less robust than it could be if there were a non-data type of start sequence.

Slave Receive Algorithm

    Figure 2.6 below shows an example Slave Receive Algorithm.  If the receive data stops prematurely, then there must also be a branch to “dump message, no reply.”  To provide the quickest possible reply, the Slave usually has to parse the message as it arrives, instead of waiting until it’s done.   Note that the Slave has to read every incoming byte and possibly just toss it.   “Can I Do This?” generally means “is the parameter that I received within an acceptable range?”

about2_6.gif (12608 bytes)

Figure 2.6 — Slave Receive Algorithm

The software that performs these functions is sometimes called a “stack”.

Data Compression

    HART makes limited use of data compression in the form of Packed ASCII.  Normally, there are 256 possible ASCII characters, so that a full byte is needed to represent a character.  Packed ASCII is a subset of full ASCII and uses only 64 of the 256 possible characters.  These 64 characters are the capitalized alphabet, numbers 0 through 9, and a few punctuation marks.  Many HART parameters need only this limited ASCII set, which means that data can be compressed to 3/4 of normal.  This improves transmission speed, especially if the textual parameter being communicated is a large one.

    Since only full bytes can be transmitted, the 3/4 compression is fully realized only when the number of uncompressed bytes is a multiple of 4.  Any fractional part requires a whole byte.  Thus, if U is the number of uncompressed bytes, and T the number of transmitted bytes; find T = (3*U)/4 and increase any fractional part to 1.  As examples, U = 3, 7, 8, and 9 result in T = 3, 6, 6, and 7.

    The rule for converting from ASCII to Packed ASCII is just to remove bits 6 and 7 (two most significant).  An example is the character “M”.  The full binary code to represent this is 0100,1101.  The packed binary code is 00,1101.  The rules for conversion from packed ASCII back to ASCII are (1) set bit 7 = 0 and (2) set bit 6 = complement of packed ASCII bit 5.

    Note that, with some exceptions, HART Slaves don’t need to do the compression or know anything about the compression.  They simply store and re-transmit the already compressed data.  Again, this is an instance where the more difficult software is placed in the device (Master) that is more capable of dealing with it.

 

Device Description Language

    As stated earlier, a HART Slave device can have its own unique set of commands.  It can also have a unique sequence of commands to accomplish some goal, such as calibration.  A Master must know about these commands and sequences, if it is to use the Slave Device to the fullest extent.  One way that the Slave Device manufacturer has of disseminating the information would be as text in a manual for the Device.  Then software engineers and system integrators could write specific code for the Slave Devices used at each installation.  Another way is to write a Device Description for the Slave Device using the Device Description Language (DDL).  The Device Description is similar to the Electronic Data Sheet (EDS) used for DeviceNet.   The HART Communication Foundation provides a specification for DDL and also provides training in how to write the DDL files.

    The Device Description is a file that can be read by a compiler, and converted into an end-user interface.  A program running in a HART Master reads the output of the compiler and is able to produce a complete sequence of menus and help screens that guide the plant engineer through whatever procedures the Slave Device can do.   In principle, using the DDL avoids writing code to talk to a given Slave Device.   Writing the DDL also forces the Slave Device manufacturer to critically examine how his device is supposed to work.

    So far it seems as though DDL hasn’t seen widespread usage, except in hand-held communicators made by Rosemount Inc.  Unfortunately, most of the software associated with DDL is apparently centered around the hand-held communicator.   In effect, the HART Communication Foundation and Rosemount Inc. still jointly distribute software needed to use DDLs.  There do not appear to be any 3rd party vendors of DDL compilers or the Master software that uses the compiler output.  We would suggest, as a remedy to this situation, that the HART Communication Foundation start giving away the DDL specification and that manufacturers of Slave Devices publish the actual DDL files via the Internet.

    The device description, using the HCF device description language, is a text file with an extension “.DDL”.  It is a series of compound statements that start with an identifying word and a name.  It looks something like this:

        VARIABLE variable_name_1
        {     structured info about variable_name_1    }

        VARIABLE variable_name_2
        {    structured info about variable_name_2    }

        COMMAND command_1
        {    structured info about command_1    }

        MENU menu_1
        {    structured info about menu_1    }

        METHOD method_1
        {    structured info about method_1    }

        etc.

        etc.

These do not imply any flow control and can appear in any order.  Each VARIABLE, COMMAND, etc. has its own structured information that must be included.  A VARIABLE is any quantity or index that is contained in the device or is used by a host to interact with the device.  In a device such as a pressure transmitter, one of the VARIABLEs (and probably the most important one) would be the pressure.  Others might be upper and lower range limits.  Another would be the device tag.  The structured information for a VARIABLE might include, for example, a format specification that tells how the VARIABLE is to be displayed to the end user.

    A COMMAND is a HART command.  The DDL has one of these statements for every HART command recognized by the device.  The structured information for a COMMAND is essentially everything related to the command including its number, request bytes, response bytes, and the returned response codes.

    A MENU is a presentation to the end user.  It can be used to present VARIABLEs or other MENUs or general information to the end user.

    A METHOD is a sequence of operations that the host is to perform on the device.  Examples would be installation or calibration.  METHODs are the least similar to the other example entities because they contain C-language statements.   When a METHOD is invoked, usually through some MENU choice that appears on the host display, the statements are executed in the order they appear.  “For” and “while” and “do”, etc. can all be used to perform looping operations.   The DDL language provides a large number of built-in functions that are essential for METHODs.  An example is “send(command_number)”, which sends a HART command.  There are also a large number of built-in functions related to aborting the METHOD.  This is essential to allow the end user to understand what is happening with the device and the host when things don’t go as planned.

    In addition to VARIABLEs, COMMANDs, MENUs, and METHODs, there are about 5 or 6 other possible entities.  These are described in HCF documents.   IMPORT is one of them that deserves special mention.  IMPORT is a means by which an existing DDL can be re-used without having to enter its entire text.  This allows, for example, the HART Universal COMMANDs, VARIABLEs, and tables, to be used by any device without having to enter them all.  IMPORT provides a mechanism for re-defining any entity in the imported DDL.  If, for example, a new field device does not use one of the Universal VARIABLEs, this can be indicated in one or two lines of code after importing all of the Universal VARIABLEs.  Perhaps the most important use of IMPORT is fixing an existing DDL.  The revised DDL is simply an IMPORT of the existing one with one or more entities re-defined.

    Among the various available HCF and Fisher-Rosemount hand-held documents, one that is seriously lacking is a document to explain how the DDL relates to what is displayed on the hand-held communicator.  In other words there is nothing that says that if I write code ‘ABC’ I will see ‘XYZ’ in the hand-held display.   Similarly, there is no way of knowing what hand-held functions (soft-keys at the bottom of  the display) become available in a given situation.  We strongly encourage Fisher-Rosemount to come up with an app note that covers this.  But until then DDL writers are probably stuck with trial-and-error.  A few general guidelines or caveats are as follows.  Keep in mind that this applies to an existing version of the hand-held and that a future version might be different.

    1.    The display is very small.  Almost all text, except for help messages, must be abbreviated.

    2.    Help messages can be quite long because the hand-held allows ‘page-up’ and ‘page-down’.

    3.    Help messages and labels are defined in the called METHOD or VARIABLE; not in the
           calling entity.  In other words, help messages associated with a given MENU are not
           defined in that MENU.

    4.    Help for a MENU is not allowed.  Thus, an end-user cannot know ahead of time whether he
           wants or needs the next MENU.

    5.    In the DDL source there is no way to define long messages on multiple lines.  To print the
           source code, it is necessary to invoke some printing method that has automatic wrap-around.

    6.    You cannot define any of the hand-held soft keys.  Everything you do must occur through
            numbered or ordered lists that you program into the display.

    7.    Format specifications in a METHOD over-ride those in a VARIABLE.

    8.    A HART communication defined in a METHOD occurs automatically.  Others often require
           the end-user to push a button labeled ‘send’.

    9.    During execution of a METHOD, there is no convenient way of having the hand-held
           repeatedly execute a loop in response to a key being held down or even repeatedly pressed.

    10.    During execution of a METHOD, an ‘abort’ will automatically be available via soft-key.
           There is no need to program this.

    11.    It is possible to define a MENU named “hot_key”.  This MENU becomes available when
            the user presses the hand-held hot key, which is one of the available function keys.  There
            are two problems associated with the hot key.  First, there does not appear to be any way
            of informing the user that the hot key is available and what it does.  Second, the hot key
    MENU is unavailable during execution of a METHOD.

    12.    Most “pre-” and “post-read” METHODS that you might want to associate with a
            VARIABLE won’t work.   You will get a “non-scaling” error message.  Apparently
            everyone has seen these and nobody knows why they happen or what can be done
            about it.

    13.    METHODs are generally not checked for errors during compiling of the DDL.  They
             don’t show up until you run the hand-held simulator.

Slave Development Steps

    Suppose you make smart (microcontroller-based) analog X-meters (where X = flow, temperature, etc.) and now you want to make a HART version of the X-meter.  Here are the steps to take.  You might also consider joining the HART Communication Foundation as a first step, since you will eventually have need of them anyway.

        1.    Make a list of things your customers do with the existing X-meters.

        2.    Of these things make a smaller list of things that are difficult to do because someone
                has to be sent to the X-meter site.  Determine whether one or a series of HART
                transactions with an X-meter could reduce or eliminate this activity.

        3.    Determine whether any existing HART Universal or Common Practice commands
                could be used to implement or partially implement these transactions.  If not, define
                one or more new commands (Device-Specific commands) that will be needed.
                Writing a DDL may help at this point by obviating missing commands.  If there are
                more than a few new commands, write a specification that spells out in detail
                (down to the individual bits) what each one does.

        4.    Determine whether the HART communication will place too much demand on existing
                X-meter resources (memory, etc.) and what kind of resources will need to be added.
                You will need EEPROM to store things like the Slave’s address.

        5.    Start hardware and software design of new HART X-meter.  Devise or otherwise
                obtain a HART Master that can be loaded with your new HART commands.  Begin
                assembling equipment for HART Conformance verification if it’s not already available.
                Or else locate an outside company that is set up to run the tests.

        6.    Decide how your customers will talk to the X-meter (DDL and hand-held master, info in
                User Manual, Complete Software package that runs on PC).  If software must be written,
                start now.

        7.    Complete the design, set up some HART networks in your own manufacturing area and
                start banging away on prototypes.  The HART Communication Foundation has code to
                help you run tests.   For example, it can send your device a message with a bit error to
                see if you catch it.

        8.    With HART X-meter apparently functioning as intended, run HART Slave Conformance
                tests.  Or have tests run by outside company.  Note:  Some tests such as bit error rate are
                of questionable value if you have followed a relatively standard design spelled out in
                application notes and have not seen any reason to suspect non-conformance.  Bit error
                rate tests are also difficult to do and are not adequately addressed in the HART
                Conformance document [2.3].

        9.    Contact HART Communication Foundation for assignment of (and payment for) a Slave
                Manufacturer’s code.   This is a byte that becomes part of the long frame address of
                every HART Slave that you manufacture.   You may not legally be able to claim HART
                compliance or use the HART trademark without this step.

        10.    Obtain other product approvals, do EMI tests, etc.

        11.    Sell more X-meters.

    If this all seems obvious, that’s great!  It means your halfway there.

Addressing Problems, Slave Commissioning, and Device Database

    The existing HART addressing scheme has several potential problems that are examined here.  Only modern (rev 5 or later) Field Instruments (using 38-bit address) are considered.  First, a Field Instrument can have any of about 275,000,000,000 possible addresses, which makes it impossible to determine the long addresses of Field Instruments after the network has been built and the Field Instruments installed.   Even if there are only 2 Field Instruments on the network and Universal Command 0 is used to try to read the long address, the procedure can still fail because short addresses might be duplicated.  This makes it almost a necessity, in all but point-to-point applications, to commission a Field Instrument prior to installation and to maintain a database of long addresses.

    Commissioning means powering up the new Slave on a bench network where it is the only Slave, and asking it for its long frame address.  (Commissioning usually includes other things as well, such as assignment of a tag.)  The long frame address is then added to a database of devices.  The database will later be used when the Slave is put into service.  Eventually, most such databases probably expand to include virtually all the Slave characteristics available through HART Universal Commands.  Initially, however, they need only contain the address or the tag for each Slave.  In most end-user applications commissioning and building the device database is probably done anyway.  Therefore, the need to do it to satisfy HART is probably not much of a burden.

    Another problem involves the unique identifier.  The first byte of the unique identifier is the manufacturer’s ID.  Since only one byte is allowed for this ID, it means that only 256 manufacturers can be so identified.  Undoubtedly, if the number goes higher than this, some change to the protocol will be devised.   Another possibility is that vendors will share an ID number and devise a serial number (last 3 bytes of unique identifier) scheme such that neither will ever duplicate the other’s unique identifiers.

    Another problem, also involving the manufacturer’s ID, is that only the lower 6 bits of this byte are used in the long address.  This means that, even though each Field Instrument can have a unique identifier, it can’t have a unique long address.  If device vendors were to begin numbering their product lines at 0 or 1 and their serial numbers at 0 or 1 (which seems entirely reasonable), then there is a pretty good chance that the long addresses might be duplicated.  The HART standards attempt to reduce this likelihood by requiring the product line byte (the second byte of the unique identifier) to be numbered in a specific way — from higher to lower numbers for half of the possible manufacturer’s IDs (first byte of unique identifier) and from lower to higher numbers for the other half.  There are also 4 ranges of product line numbers and 4 ranges of manufacturer’s IDs.  Each range of manufacturer’s ID numbers has a specific range of product line numbers.  This is illustrated in Table 2-2 below.

Manufacturer ID Range Range Of Acceptable Product IDs
0 to 63 (decimal) Start at 0 and increase
64 to 127 Start at 239 and decrease
128 to 191 Start at 127 and decrease
192 to 255 Start at 128 and increase

Table 2-2  —  Allowed combinations of MFR IDs and product line numbers

    Finally, the addressing scheme creates a need for device vendors to register their devices with a registration body — the HART Communication Foundation.   This is a costly bookkeeping adventure that could just as well be done by end-users.  The end-user already decides which vendor’s device to buy, maintains an address database, and assigns a tag to each device.  He could as easily assign the address and determine which DDL he needed, based on the device vendor and serial number.

Bell-202:  Bad News in Europe

    Telecommunication systems (phone utilities) use certain well-defined tones for administrative purposes.  These tones fall within the voice band but are only effective if there is no energy present at other frequencies.   The tones used in Europe are different from those of the United States.  Unfortunately, one of those used in Europe falls into the range of 2130 Hz to 2430 Hz (see [2.4] for example).  The Bell-202 frequency of 2200 Hz could appear as a pure tone within this forbidden band.  Consequently, if Bell-202 equipment were to find its way onto a European phone grid, it could cause problems.   Normally, European telecommunication regulations prevent the sale and distribution of incompatible equipment, so that this doesn’t happen.

    So what does this have to do with HART?  If HART communication is confined to private industrial networks, as is usually the case, then there is no association between HART and telecommunications.  HART should be just as acceptable in Europe as anywhere else.  It should, but it isn’t always.   We have experienced problems when using the term “modem” in instruction manuals, etc.  Apparently this term has become almost universally associated with telecommunications.  And in some instances this has led to seizure of HART equipment by authorities who did not understand its purpose.  We found that we had to remove the term “modem” from some literature.

    Under certain circumstances it is possible to do HART communication over European phone lines.  This is further discussed in the section entitled HART Bridges and Alternative Networks.

 

Grounding and Interference

    An industrial environment can produce a variety of powerful electric and magnetic fields, as well as significant voltages between different “grounds”.  Most often they are at 50 Hz or 60 Hz and don’t pose much of a threat to HART.  However, sometimes they or their harmonics fall into the HART band (about 900 Hz to 2500 Hz).  Since HART signals have a rather small amplitude, there is a possibility that these higher-frequency fields will disrupt signaling.   Interference that would have no effect on an analog 4-20 mA signal (because of lowpass filtering in the controller) might be enough to destroy HART messages.  Here we look at how to protect against the interfering fields and how ground potential differences can still manage to cause trouble.

    The circuit of a HART Field Instrument is typically contained inside of a metal case that is at earth ground.  The circuit is isolated from the case, except for feedthrough EMI filters and an inevitable small capacitance from the circuit to the surrounding case.  There is no single wiring scheme that is best at reducing interference in all circumstances.  But for Field Instruments of the type described, the following is usually recommended:  The Field Instrument and Controller (Master) are connected by a shielded twisted-pair cable.  The shield is grounded at the Controller end and left open at the Field Instrument end.  The shield prevents electric fields from coupling into the signal conductors, while the twisting attempts to reduce the effects of magnetic coupling by forcing equal coupling into both sides of the pair.  The shield is left open at the Field Instrument end to avoid the conduction of ground currents.  A ground current on the shield couples magnetically to the conductors of the twisted pair.  These ideas are more thoroughly explained in [2.5].

    A ground current can result from making a connection between two “grounds” that are at different potentials.  The different potentials can arise from huge currents (amps) flowing through cables that separate the grounds.  A ground current also arises when a conductive loop is formed in the presence of a strong (varying) magnetic field.

    Even when the wiring rules are followed, this difference in ground potentials can cause interference.  To see this, consider the circuit of figure 2.7.   The cable shield is connected at the Controller ground, according to recommended practice.  A voltage Vin exists between the Controller ground and Field Instrument ground.  The circumstances that create Vin usually also cause a relatively large impedance in series with it.  This is represented by Zs.

wpe8.gif (6074 bytes)

Figure 2.7 — HART Circuit Showing Ground Potential Difference

At HART frequencies we can make the following assumptions:

1.    The cable can be considered a lumped circuit consisting mainly of capacitance.

2.    The Circuit-to-case Capacitance inside the Field Instrument is negligible.

3.    The impedance of the box labeled “Circuit” across the Field Instrument terminals is practically infinite.

Note that one side of the twisted pair is grounded at the Controller.  Then the equivalent circuit is as in figure 2.8.

about2_8.gif (3981 bytes)

Figure 2.8 — Equivalent Circuit for Effect of Ground Potential Difference

The interference is the voltage Vout developed across the Field Instrument terminals.  It is related to Vin by

If we assume that the EMI feedthrough filters have nearly equal capacitance so that C1 = C2, this becomes

Quite often Zs is large (small capacitance, large resistance).  Then sC2Zs >> 1 at HART frequencies and the expression becomes approximately

In a properly constructed HART network the pole frequency in this latest expression will be above the HART band so that within the HART band the expression reduces to

As an example, suppose that Zs is due primarily to a capacitance with a value of 1000 pf.  And let Vin = 100 volt at 2 kHz, R = 250 ohm.  Then the magnitude of Vout = 0.31 volt.  This would certainly destroy the HART signal. 

    One way to remove the effect of Vin is to connect the cable shield to the instrument case at the instrument end.  This gets rid of or reduces the effect of Vin, but may cause other problems.  Some experimenting may be necessary.  A better way is to use a separate ground conductor between the controller and instrument grounds.  This new ground conductor can be separate from the network cable.  Or it can be built into the network cable.  Special cables, having both an inner and outer shield, are made for this purpose.

HART and Intrinsic Safety

    HART was always intended to be retrofitted to existing process loops, including those that are intrinsically safe (IS).  The relatively low HART signal level and superposition of the HART and analog signals are the result.  Also, an individual HART Field Instrument is not too different from a non-HART Field Instrument in terms of power consumption and equivalent capacitance and inductance.   If we look only at signaling and Field Instrument design, we might conclude that combining HART with IS is not a problem.  But when we look at HART in its broader context, there are problems.  They are, or result from, (1) the need to transmit HART through IS barriers, (2) topology differences between HART networks and conventional current loops, and (3) the existence of a new device — the hand-held communicator.  The implications of combining HART and IS and their effect on each other will be considered next.

    The simplest form of  IS HART network is one with just a single 2-wire Field Instrument (a Point-to-Point HART network).  The Field Instrument and cable are located in the hazardous area and a barrier separates the safe and hazardous areas.  This is illustrated in figure 2.9.

about2_9.gif (3037 bytes)

Figure 2.9 — Simplest IS Arrangement

We assume for the moment that nothing else will ever be connected to the network at the hazardous side.  However, there may be a HART Master at the safe side.  There are two things to consider here:

    First, the barrier, Field Instrument, and cable must represent a compatible combination.  That is, the Field Instrument must consume less voltage and current than allowed by the barrier and the cable+Field Instrument must represent less C and L than allowed by the barrier.  A HART Field Instrument consumes about the same voltage and current as a non-HART Field Instrument.  And the addition of HART components (such as receive filter, etc.) don’t add much C or L.  Thus, the single HART Field Instrument is comparable to a conventional process instrument and presents no unusual difficulties in achieving IS or in compatibility with other IS components.

    (Note:  We will neglect heating effects on small components, even though this can be an important consideration in the design of the Field Instrument.  We assume that the added components needed for HART affect IS only through added C or L or changes in circuit topology.)

    Second, the barrier must pass the HART signal with a minimum of attenuation and distortion.  Barriers can affect the HART signal in a variety of ways, depending on the type of barrier and on associated components.  The HART Physical Layer Specification prescribes several tests that barriers must satisfy.   These tests are all related to insuring that there is sufficient signal passed in each direction through the barrier; and that the signal is not distorted by the barrier.

    In a conventional resistor-zener diode barrier, it is theoretically possible to operate too near the zener clamp voltage so that the HART signal excursions are clipped.   Usually, however, the peak HART voltage is on the order of 0.25 volt; and barrier ratings are conservative enough that clipping doesn’t occur.  (Note that if the barrier working voltage is too near the clamp voltage, there would be too much leakage current.  Analog signaling would become inaccurate.)  Another effect of a resistor-zener barrier is a flat attenuation of a voltage signal.  A barrier with a 300 ohm resistance, used with a 250 ohm current sense resistor, creates a divider that will attenuate by 0.45.  The Controller will see a HART signal voltage that is 0.45 of the voltage across the network.  Yet another effect of the standard resistor-zener barrier is a lowpass filtering caused by junction capacitance of the zener diodes.  Capacitances of several thousand pf are possible.  However, this often just adds to a much larger cable capacitance and doesn’t need to be taken into account.

    An important consideration for active or repeater barriers is that they do not remove the HART signal with a lowpass filter designed primarily to pass the analog 4-20 mA signal.  Another is that they do not chop (to create AC) at a frequency that is in or near the HART band.  These special barriers must often be specifically designed to work with HART.

    The next step up in complexity is an IS HART network identical to that described previously, except that it contains two or more multi-dropped Field Instruments, as illustrated in figure 2.10.

abou2_10.gif (3217 bytes)

Figure 2.10 — More complex IS Arrangement

Again, the barrier, cable, and Field Instruments must be a compatible combination.  Since more L and C is being added with each Field Instrument, the likelihood of a viable combination becomes less as their number increases.  In fact, we’ve not heard of any instances of two or more Field Instruments being used in this way.   But this may simply reflect the facts that (1) most HART applications are point-to-point (one Field Instrument) instead of multi-drop; and (2) most applications are not IS.  If the parked current of each of the Field Instruments is 4 mA, and if equivalent L and C are kept sufficiently low, it should be possible to operate about 4 or 5 Field Instruments from a single conventional barrier.

    Finally, the most complex situation results when a hand-held communicator (HHC) is connected to the hazardous area network, as illustrated in figure 2.11.  (In theory there could still be more than one Field Instrument, as in figure 2.10.   However, the presence of the HHC multiplies the IS difficulties such that the prospect of multiple Field Instruments becomes less likely.)

abou2_11.gif (3743 bytes)

Figure 2.11 — Most complex IS Arrangement

Instead of a HHC, the added device might instead be a second Field Instrument capable of actively generating its own voltage or current signal (such as might occur in a 4-wire Field Instrument).  Either type of device presents the same kind of problems with respect to IS.

    The HHC (or active Field Instrument) signal is AC-coupled to the network and is infallibly clamped so that peak voltage can’t exceed about 1 or 2 volt.   Using the HHC means that the instantaneous network voltage under fault conditions can now equal the sum of the barrier clamp voltage and the HHC clamp voltage.   For a 28 volt barrier, the total voltage then becomes 29 to 30 volt.  (The reason that the maximum voltage isn’t still identical to the barrier clamp voltage is that there is resistance between the barrier zener diodes and the HHC.   This resistance is the combination of the barrier resistance and cable resistance.)   There is also an increase in current that could conceivably be drawn from the combined barrier and HHC.  The barrier and HHC may be thought of as two power sources and two barriers, both supplying power to the network.  Since the HHC signal is an AC voltage and current with average value of zero, and since its peak value is quite small compared to a 28 volt-rated barrier, some safety agencies ignore it.  Others don’t.

    Suppose that we are stuck dealing with a safety agency that assumes the extra voltage and current.  What are the ramifications?  Then even if the same barrier is used in figures 2.9 and 2.11, a Field Instrument that was acceptable in figure 2.9 might not be in figure 2.11; because of (1) the higher voltage and current and (2) the extra capacitance introduced by the HHC.  Not only might the Field Instrument be unacceptable, but the “entity” concept of constructing the IS system no longer works.  The barrier ratings no longer mean anything and the combination of barrier, Field Instrument, and HHC may have to be certified as a unit.   This is not a trivial consideration, since it means that a prospective customer may be locked into a combination of devices from a single vendor.

      NOTE:    The IS design of the HHC itself is a technical tour-de-force.  The HHC is generally battery-operated and isolated from the network by an infallible transformer.  The circuitry on the HHC side of the transformer includes the battery.  Without appropriate IS design, there would be ample opportunity for gas ignition, irrespective of any connections to the outside world.  The network side of the transformer is subject to the full current allowed by the barrier and must be sized and clamped appropriately.  And, as indicated earlier, the output must be clamped so that only 1 or 2 volts can be produced at the terminals.  And on top of all of this, there’s that pesky expectation that the HHC still provide HART communication!       Currently we know of only two manufacturers of Intrinsically Safe HHCs:  Rosemount Inc. and MTL.

    One way around the situation just described is to have the HHC interface be similar to that of a passive 2-wire Field Instrument; so that the combination of the Field Instrument and HHC appear to be just two Field Instruments as in figure 2.10.  But this requires that the HHC draw current from the network — an unacceptable approach when analog signaling is present.

    The entity approach to IS and the use of HHCs in hazardous locations are both such important concepts, that we probably have not heard the last of this apparent clash.  It seems likely that some kind of compromise can be worked out so that both concepts are available world-wide.

    In conclusion, HART was originally conceived to be IS-compatible and is generally well suited to use in IS environments.  Multi-dropped Field Instruments in an IS environment are possible but not often used.  An HHC in the hazardous area is possible, but presents special problems.

HART and CE Mark

    In 1996 the CE Mark became mandatory for electrical/electronic products sold in Europe.  The CE Mark means, among other things, that the product satisfies specific requirements for ESD immunity, EMI immunity, and EMI generation.  For instrument makers EMI immunity usually surfaces as the major concern.  The specification for EMI immunity that usually applies to HART devices is [2.6].  This, in turn, refers to [2.7].  The test signal ranges from 150 kHz to 80 MHz and is modulated with 1 kHz.

    A HART Field Instrument can be affected by EMI in two ways:  (1) Analog Signaling may become inaccurate and (2) HART Signaling may suffer an increased error rate.  The instrument vendor is allowed to state what constitutes a failure of the device in the presence of EMI.  For analog signaling this usually isn’t difficult.  For example, the vendor can state that the accuracy remains within X % of span during exposure to EMI.  For HART signaling, however, we are not aware that any widely accepted criterion has been established.  The likely affect of EMI on HART communication would be to interfere with it and increase the bit error rate.  This, in turn, increases the message error rate.  The increase in message error rate could be so small as to go unnoticed by an user; or so large as to completely stop the HART communication.  Here we will examine how EMI can affect the HART Field Instrument and look at some possible tests and failure criteria.

    A typical 2-wire process instrument has its circuit contained in a metal case that is connected to earth ground.  The circuit is isolated from the case and is connected to an external cable through EMI feedthrough filters, as illustrated in figure 2.12.  The cable shield or one of its leads is grounded at the end opposite the process instrument.

abou2_12.gif (5985 bytes)

Figure 2.12 — RF Circulating Current In Process Instrument

When this arrangement is exposed to EMI, a circulating current is established.  Its path is shown in the figure.  For the purpose of looking at the RF current path, all of the cable conductors are lumped together as though they just one single conductor.  The process instrument circuit is also just one single piece of conducting material, with a small capacitance to the case.  Ideally, most of the circulating RF current will go through the EMI filters and instrument case, bypassing the circuit.  But some of the current will go through the circuit and couple into the case.  The latter current takes various paths through anything it can find on the circuit board or boards.  It can be amplified if it encounters an active circuit with a high enough bandwidth.  But it usually encounters some nonlinearity (transistor junction, for example).  This demodulates it and brings it down into the low frequency range, where it may cause offsets and amplifier saturation.  The ENV50141 test is particularly bad because it uses a 1 kHz modulating signal — a frequency within the HART band.

    Another effect that can be equally bad results from the EMI filters being unmatched.  That is, one of the EMI filters is slightly different from the other.  Or, even if the EMI filters are matched, the impedance of the circuit to earth ground might be different at one terminal than at the other.   The circulating RF current then causes a normal-mode RF voltage to appear across the input terminals.  Depending on frequency, this voltage may be amplified by circuits or may be subjected to nonlinearities as previously discussed.  Ways of keeping the RF out of the circuit are discussed in [2.5].  But we still need a test criterion.

  A possible test — one that we have used —  is to have a known message sent back and forth repeatedly between two devices, one of which is exposed to the EMI.  Message errors at both ends are counted and if a threshold is exceeded, the device has failed.  The problem with this is that devices aren’t necessarily designed to operate in this manner.  Modifying them or tapping into their hardware in some way negates the test, since the device tested in this way isn’t the same as the device to be sold.  Modifying just the software might be OK.  If there is enough code space available, then a test mode that works as described might be possible.

    A test that can be done on a Field Instrument is to repeatedly send it a Universal Command (such as command 0) and get its reply; with the Field Instrument located in the EMI Field and the Master located outside of the Field.   If the message received by the Field Instrument is in error, then either it won’t send a response or it will send a response with an error status indication.  Either way, the Master knows something went wrong.  This testing assumes that messages reaching the Master are OK and is a test primarily of the ability of the Field Instrument to receive in the presence of EMI.  The cable between Master and Field Instrument should be passed through the EMI containment wall using EMI feedthrough filters so that the EMI doesn’t reach the Master.

    Assuming that the command-reply type of test is used, then what should be the criterion for failure?  It might be nice to know the number of bit errors, because this relates to the probability of an undetected message error (UME).  But the HART Master doing the error monitoring can only count transaction errors.  That is, if it sends a command and gets a good response back, then everything’s fine.  But if it gets no response or a bad response, there is no way of knowing how many bit errors may have contributed.  Consequently, we have to look at some less informative but more practical criterion.  Real devices will probably show one of the following 3 behaviors, with the first two being the most likely:

            1.     There are no transaction errors logged.

            2.     There are massive numbers of transaction errors as the interfering frequency
                    is swept through specific regions.

            3.     There are a few transaction errors that aren’t always repeated on subsequent
                    frequency sweeps.

Most of us would agree that if (2) happens then its back to the drawing board.  But either (1) or (3) would mean that we still have an usable HART system.   Therefore, we suggest as the criterion that there be no interfering frequencies or frequency bands in which the number of transaction errors is observed to steadily increase at a rapid rate.  That is, there can be no interfering frequencies or bands in which there is essentially no HART communication.

    As a practical matter, when there are few or no communication errors, the examining technician must be convinced that the devices really are communicating and that the errors, if any, will be logged.  A convenient way to do this is to loosen the connections to a device and wiggle the wires enough to cause momentary disconnections.

    In the standards cited above, “RF” starts at 150 kHz.  Unfortunately, this is low enough that it will pass through conventional EMI feedthrough filters, most of which are designed to start having an effect at 10 MHz or more.  Not only does the test start at a relatively low frequency of 150 kHz, but the ENV50141 specification seems particularly unfair and unrealistic in requiring the RF source to be connected directly to the HART network during the test.  That is, instead of the network being an antenna for the RF, as would happen in reality, the network becomes part of a lumped circuit that contains the RF generator.  The consequence is that a very large common-mode signal at 150 kHz and modulated with 1 kHz gets applied to HART receive circuits.  We are not aware of any HART Field Instruments that pass this test.  It seems likely that a future revision of the standards will provide a more realistic test that does not call for any actual connection of the generator to the HART circuits; and will reflect the difficulty of RF coupling at such a low frequency.

 

Electrical Measurement of a HART Network

   Suppose you want to see the HART signal as in figure 1.3.  You can do this with an oscilloscope.  But if you plan to connect it across the twisted pair, you may want to use differential probes instead of a conventional probe.  The ground connection of the conventional probe may create an undesired circuit path.  For example, if the network circuit is that of figure 1.2, the conventional probe may either short-circuit the power supply or the current-sense resistor.  Using large capacitors between the twisted-pair conductors and the conventional probe leads doesn’t help.  It provides DC isolation, but you may still be short-circuiting the signal that you’re trying to view.  Using a conventional probe with a floating, battery-operated scope is OK.

 

Isolating A Non-Isolated Modem

    A need sometimes arises to galvanically isolate a HART Master from the HART Network using components that are easily obtained.  This can be done as in figure 2.13.

wpe41.gif (3904 bytes)

Figure 2.13 — Isolating HART Master

Choose an electrolytic capacitor that has low leakage current and avoid using it high temperatures.

 

Troubleshooting:   What To Do When “It Just Won’t Talk”

    Whether you’re an end-user, a systems integrator, a programmer, or a hardware person; you’ve probably had occasion to use a few choice words when a HART device just won’t talk.  Often you can get somebody else to figure out why.  But in case you’re elected, here are some things to look for.  We will assume the common situation in which we are trying to talk to a HART Slave Device that is currently in production and whose health is not suspect.

Easier Things to Try

1.    First, is the wiring correct?  Is the Slave powered?  If it is 2-wire, does it have sufficient DC operating voltage and current?   Is the Slave connected across the combined power supply and current sense resistor (this works); or across only the power supply (this doesn’t work)?

2.    Is it the only Slave on the network?  If there are two or more, do they have different polling addresses?  (Two Slaves with the same polling address can result in a collision and the appearance that neither exists.)

3.    If the Master is a personal computer + modem, are you sure that you have the correct COM port selected?  If you’re using a multi-tasking operating system, is it capable of sending out the bytes without significant gaps?  (See section entitled   HART and PCs.)

Less Easy Things

4.    If possible connect an oscilloscope (a differential connection is usually best, if possible) across the network.  Set it to AC-coupled and band-limit to 20 MHz or less.  When the network is supposed to be silent (nobody talking) you should see just a noise level of something like 0 to 20 mV p-p.  A larger amount of interference at 50 Hz or 60 Hz is probably still OK.   If there is interference greater than 20 mV p-p in the region of 900 Hz to 2.5 kHz, this could be the problem.  You may need to find and eliminate it.

5.    With the oscilloscope across the network, can you see bursts of carrier each time you take action to look for or talk to the Slave?  A Master will usually try several times so that you see the carrier burst repeated about once or twice per second until the Master gives up.  If you don’t see these bursts at all, then the Master is suspect.  If you are developing the Master software, are you writing to the correct port?  Are you turning on RTS (request-to-send) during the transmission?  Is the bit rate set correctly?

6.    If you do see the bursts, examine a burst carefully.  It should consist of two parts:  the Master transmission followed by the Slave transmission.  These will usually have different amplitudes.  There may be a gap between them.  The Slave transmission will usually last longer.  If you see only one part, it is the Master transmission and the Slave transmission is missing.  Examine the Master transmission in greater detail.  One of the nice things about HART is that you can read the transmitted bits from the waveform.  Are there at least 5 preamble characters being generated?  If not, then are you turning ON RTS too late?  If there are enough preamble characters; check the start delimiter, address, and so on through the checksum.  The sequence of characters for sending Universal Command 0 to a normal (unparked) Slave is

        FF FF FF FF FF 02 80 00 00 82

The bit sequence is START, LSB, …., MSB, PARITY, STOP.  If part of the checksum is missing, are you leaving RTS ON long enough?  If the Slave is an older type of device, and you can specify the number of FFs, then increase them from 5 to 20.  Is the parity correct?  For each FF it should be a ONE.  Make sure it’s not ZERO or missing.  Check the length of the Master’s message.  For the 10-byte sequence shown above, it should be 91.67 millisecond.  If it’s off by about 833 microsecond, or a multiple of 833 microsecond, then there are extra or missing bits.   Is the amplitude of the Master transmission large enough?  It must be at least 120 mV p-p to insure proper carrier detect operation in the Slave.  Amplitudes of 300 mV p-p to 500 mV p-p are more typical.

7.    If there are two distinct parts to each burst, it means that the Slave is replying but the Master is not seeing the reply.  Do the same thing for the reply portion of the burst as described in (6).  That is, try to identify the sequence of bytes that the Slave has generated.  Is there a distinct preamble with at least 4 identifiable FFs?  If not, are you turning OFF the Master’s RTS too late?  Is the Slave signal amplitude too small?  It must be at least 120 mV p-p and is usually more like 250 mV p-p.

 

 

HART Repeater

    A repeater may be necessary when the desired cable length exceeds limits set by the HART Standards or when there are more than 15 devices.  It is a two-port device placed between two network segments.   From a Protocol viewpoint it makes the two segments look like one network.   Although the repeater might also be equipped to repeat the analog 4-20 mA signal, our discussion here is limited to a device that repeats only HART signals.

    To preserve the HART timing, a repeater must repeat in real time.  That is, it cannot store messages for later forwarding.  Delays must be limited to a few bit times if various timers are to work reliably.  Another limitation is noise.  A repeater cannot simply amplify and re-transmit the FSK signal, since this would also amplify and re-transmit noise on the network segment.   This narrows the choice of repeater architecture to one in which the incoming signal is demodulated and then re-modulated.  In addition, we must re-modulate with a “clean” signal.   The output of a demodulator will contain jitter due to noise and to the demodulation process.  This jittery signal should not be applied directly to the re-modulator, since this would result in a degraded signal to one or more receiving devices.  Instead, the demodulator output should be detected in UART fashion (i.e., sample at mid-bit).  Some logic is also needed to determine at start-up which bit is a start bit and to count out each 11 bits that have passed and identify the next start bit.  A block diagram of the repeater is shown in figure 2.14.

wpe42.gif (5280 bytes)

Figure 2.14 — Repeater Block Diagram

    Notice that, except for the line interface circuits and carrier detects, there is just a single signal path that is turned around as needed.   The direction can be determined by a relatively simple state machine as illustrated in figure 2.15.

wpe41.gif (4866 bytes)

CA = Carrier Detect on Network A
CB = Carrier Detect on Network B

Figure 2.15 — Direction State Machine

    There are 3 states:  idle, B>A (Network B to Network A), and A>B (Network A to Network B).  Idle means that both ends are listening and the driver switch is not connected to either network.  CA, CB = 1,0 means that there is carrier at network A and not at network B.  And so on.  If both carriers are present, the last state is retained.

    A problem with all interface or bridge devices is the time it takes to turn the line around (or to turn on a signal path).  This is usually related to carrier detect and can often be done in less than one character time.   However, the loss of a character increases the number of preamble characters that may be lost from 2 to 3 (see also Part 2:  Startup Synchronization in HART).   If only 5 preamble characters were sent, as is often the case, this leaves only 2 as valid preamble.  Thus, the margin against missing the preamble is reduced.   If another device, such as a 2nd repeater were to be included somewhere in the network, there would likely be frequent failures to recognize the start of a message.  The change to a HART Slave to force it to require more than 5 preamble characters is usually minor.  Therefore, the vendor of the Slave device may be willing to increase the preamble size for the device in the interest of satisfying a customer.  At the Master end the software can be changed so that it always uses a preamble of greater than five characters, ignoring whatever number the HART Slave says it should use.

 

HART Gateways and Alternative Networks

    Conventional HART, operating at 1200 bits/second and using a process loop as a network, has been the focus throughout most of this book.  However, the desires of HART process equipment customers are seldom so limited.  The need arises to connect
HART devices in unconventional ways, which is the subject of this section.  These unconventional methods can be divided roughly into two categories:  those that still use HART protocol or some of it, and those that connect HART with networks using entirely different protocols.  An interface between networks having different protocols is called a Gateway [2.8].  Examples would be HART to Devicenet [2.9], HART to Ethernet, HART to Modbus, etc.  Some of these unconventional methods are presented here, in no particular order.

 

    PC as Gateway

    About the easiest way to form a Gateway is with a personal computer.  This is sometimes done by systems integrators who need something up and running in the shortest possible time.  As personal computers become less expensive and more reliable, this
becomes more of an option.  Small, inexpensive, single-board computers that implement DOS or Windows CE can also make this a reasonable approach.

    As an example, suppose you need an Ethernet-to-HART gateway.  This is done as in figure 2.16.

Figure 2.16 — Using PC As Gateway

You buy the 3 pieces of hardware and write the software.  For applications that are more cost-sensitive or that require greater reliability, a dedicated piece of hardware may be needed.  Some of these are examined as follows.

 

   DeviceNet to HART

    DeviceNet is becoming the de facto standard for high speed on-off sensing and control.  HART and DeviceNet have little in common, as indicated in the following table.  We wouldn’t expect many similarities, since the two protocols are intended for different purposes.

Network Type HART DeviceNet
Modulation Method FSK Baseband with Bit Stuffing
Data Rate 1200 BPS 125 kBPS and up
Application Transmit text and floating pt. numbers Transmit discrete I/O (on-off, open-closed)
Power and Signal on Same Pair? Yes (2-wire is possible) No (uses 4 wires)
Number Addressees per network 15 or 275,000,000,000(*) 63
Equality of Devices Master and Slave Devices equal but prioritized
Message Frame HART (UART-based) CAN
Possible Message Length Long Short but can be continued
Device Power Available Milliwatts to Watts Watts

(*) HART allows only 15 devices on a conventional current loop-type of network. 
But there are 275e9 possible addresses.

Table 2.3 — HART and DeviceNet Comparison

Suppose, however, that someone implementing DeviceNet needed to read the process variable of a HART flow meter?  A dedicated gateway between the two networks might be a possible way.  It might work like this:  At the DeviceNet side, the gateway looks like a DeviceNet Server (produces response) with Cyclic I/O Messaging at perhaps about once per second.  At the HART side it appears to the flowmeter as a HART Master.  Once each second it queries the flowmeter to get the process variable at the HART side and then transmits this variable to all consumers at the DeviceNet side.  At power up, the gateway device would go through the DeviceNet configuration, receive its assigned DeviceNet address, and become a publisher of information.  Then it would determine the address of the HART flowmeter in preparation to read the process variable.

    A dedicated gateway would probably be designed to work with more than one HART Field Instrument and would publish the process variable corresponding to each Field Instrument.

 

    HART Over RS485/RS232

    Conventional HART uses FSK modulation to translate the frequency spectrum to a region that is compatible with 4-20 mA.  In some applications where there is no current loop, the modulator and demodulator are simply removed and HART is transmitted as a baseband signal.  This is illustrated in figure 2.17 for RS232 and RS485 line drivers and receivers.  RS232 is more suited to communication between just two devices, while RS485 allows the construction of a network of several devices.  RS232 is limited to a distance of 50 feet (15 meter), per the standard.  RS485 allows up to several thousand feet (one or two km).

Figure 2.17 — Baseband HART Using RS232/RS485

    In both of these arrangements the message generated by the HART device is the same series of 11-bit characters that would normally be sent to a HART modem.  The bit rate can be 1200 bits/second as in conventional HART.  Or it may be higher.

 

    Combined Baseband and Conventional HART 

    A combination of baseband RS232/RS485 and conventional HART signaling is also possible.  When RS485 is used, it is possible to build a super HART network as illustrated in figure 2.18.

abou3_20.gif (6929 bytes)

Figure 2.18 — Super HART Network Using RS485

This super network can have 31 bridge devices (the limit for RS485) and 15 HART Field Instruments per bridge device, for a total of 465 Field Instruments.   Except for line turn-around time in the Bridge, all HART timing is preserved.  The considerations for the bridge device are similar to those for a repeater.

Telecom HART

    Since the HART signal band is essentially the same as the band available to voice signals in telephone networks, a telephone network can be used to transmit HART.  In the United States and Canada the HART FSK signal frequencies are OK as is.  In Europe or other countries that use CCITT standards, HART can still be used except that the frequencies must be changed to 1300 Hz (logic 1) and 2100 Hz (logic 0).  These frequencies are acceptable in the U.S. and Canada.   And, fortunately, most HART and Bell-202 modems will accept these two frequencies.   This leads to some simplification in an interface device. 

    A typical HART-over-phone-lines application is shown in figure 2.19.

abou3_21.gif (4157 bytes)

Figure 2.19 — Illustration of Using Telephone Network for HART

The computer and office modem constitute the HART Master.  The office modem is a standard Bell-202 or CCITT V.23 telecom modem.  There is no point in trying to adapt a HART modem at the office end, since commercially available telecom modems already have the desired certification and are directly compatible with the telephone network.  They just need to be set up to work with HART.  This is explained in more detail in a HART Application Note.

    When used with the Public Switched Telephone Network (PSTN) the computer and office modem can “call up” any number of Field Instruments.  The size of this network is virtually unlimited.  And, of course, there can still be up to 15 HART Field Instruments served by each Telecom-HART Interface, so that there can be up to 15 Field Instruments at each phone number.

    The “Telecom-HART Interface” is a necessary part of the scheme, since a process instrument isn’t directly compatible with the telephone network.  Even when a leased line is purchased from the phone company, direct connection of a process instrument isn’t advised because signal levels and impedances will not be correct.  If the process instrument is a 2-wire device, there is also the question of how to power it.  The Telecom-HART Interface provides the functions of figure 2.20.

abou3_22.gif (6621 bytes)

Figure 2.20 — Block Diagram of Telecom/HART Interface

The Interface consists of two signal paths (toward and away from the Process Instrument) and some logic (or microcontroller) to decide which path is active.   Deciding which path should be active is more than just sensing carrier detect.   Once a path is closed carrier appears at both ends.  Therefore, some form of state machine is desired.  One possibility is that both paths are normally turned OFF.  When both carriers are absent, the device goes to this (both paths OFF) first state.   If a telco carrier should become present and HART carrier is absent, the upper path is switched ON (second state).  If a HART carrier appears and the telco carrier is absent, the lower path is switched ON (third state).  If both carriers are present, the device simply maintains the last state (either second or third).

    At the start of a transaction the telco carrier will come ON and the upper path will become active.  As soon as the path becomes active, both carrier detects will be on and the upper path will remain active.  The Master will finish its transmission so that the telco carrier goes away.  At this point, the carrier at the HART side might also go away if there is no immediate reply by a HART Slave.  Then both paths would become inactive.  When the Slave finally replies, there will be a HART carrier and no telco carrier so that the bottom path will become active.  Another possibility is that after the telco carrier stops, the HART side carrier stays active because the HART Slave has already begun to reply.  Then the bottom path will be made active at the same time that the upper path becomes inactive.

    At the telephone end, the interface device provides a data access arrangement (DAA), so that the device may be legally connected.  Limiters in both paths control the amplitude of signals that are applied to the respective networks.   An entire modem is added if the device is to be used in Europe.  This modem accepts the HART signal frequencies of 1200 Hz and 2200 Hz and converts them to CCITT-compatible frequencies of 1300 Hz and 2100 Hz.

    A potential problem with trying to use conventional Master software in this telephone application is the delays in the telephone network.   When the Master sends out its command, it arrives at the interface device some time later.  The return trip is similarly delayed, so that Master software may time out, thinking that the Slave didn’t reply.  As much as half a second is possible, though not typical.  The Master software should be designed to take this into account.   Clearly, burst-mode, Master arbitration, and other conventional HART activities dependent on timing are probably impossible in this application.

    In this application (and probably others), the possibility of inadvertently turning on burst mode in a device must be carefully considered.  It is easy to turn burst mode on.  But because the network doesn’t support conventional HART timing, it may be impossible to turn burst mode off without a trip to the site of the bursting Field Instrument.  If this is a great concern, then it may be necessary to incorporate a micro-controller into the Interface and screen (filter) the HART commands.  But this greatly complicates the Interface and discards the convenience of a modulation method that is already compatible with phone lines.   A more reasonable approach is probably to control the Master software so that it never issues commands that would activate burst mode.

 

   Fiber Optic HART

    By combining inexpensive and powerful laser diodes (emitter) with efficient photodiodes (detector), enough power (about 4 mA at 12 volt) is produced to operate a parked HART Field Instrument.  The result is a point-to-point HART network with only optical fibers connecting the Field Instrument.  The scheme is illustrated in figure 2.21.

abou3_23.gif (2700 bytes)

Figure 2.21 — Fiber Optic HART Network

Everything except the Optical Fiber and Field Instrument are located in the control area.   A special interface built into the Field Instrument converts a conventional 2-wire HART Field Instrument to an optical Field Instrument.  This equipment and the services to retrofit Field Instruments are available from NT International [2.10].

 

    Single Modem/Multiple Point-to-Point

    A common situation is that of a HART user who has only point-to-point networks (one Field Instrument per network or per current loop) and wants to use one computer to talk to all of them.  The solution that comes to mind most quickly is to use multiple modems.  In fact, this is probably the only solution that is able to maintain the full protocol, including arbitration.   But, if we know that there will not be a need for the full protocol, another possibility is to switch a modem from one network (process loop) to the next.    The problem with this is how to do the switching.  Electromechanical relays probably aren’t the answer.  Semiconductor switches might create too much leakage current.  Another problem with switching is the need for the Master device to maintain a table of network  and device addresses.  That is, the Master must remember not only the Field Instrument address, but the network of that particular Field Instrument.  A possible solution that doesn’t involve switching and maintains the reliability and integrity of each process loop is that of figure 2.22.

wpe44.gif (9186 bytes)

Figure 2.22 — Single Modem Coupled To Multiple Point-to-Point Networks

Here, the individual networks are transformer-coupled to a single modem.  The modem is specially designed to have an impedance of zero or nearly zero, whether transmitting or receiving.  Zero impedance during receive serves two purposes:  (1)  It allows the modem to collect all of the signal from one of the Field Instruments instead of having the signal distributed (lost) to other networks.   (2) It relaxes transformer requirements by increasing the associated L/R time constant.   The coupler device is entirely passive and galvanically isolates the network from the modem.  The schematic of a single coupler is shown in figure 2.23.

wpe42.gif (2948 bytes)

Figure 2.23 — Coupler

The coupler is a 3-port device that can easily be designed for DIN-rail mounting.   Its resistance from controller port to Field Instrument port is very low, so that very little voltage drop is introduced into the loop.  There is also complete galvanic isolation of the current loop from the modem.

    Using the scheme of figure 2.22 one modem communicates with up to 15 networks (15 Field Instruments).  When the modem transmits, the transmission is seen by all of the Field Instruments and only one replies.  When the modem is receiving it must sense the current flow through its terminals.

    Some of the considerations in applying this method are:

    1.    The drive capability of the modem must be quite high, since it is sees the combined loads represented by
            up to 15 networks.

    2.    The modem must maintain a low impedance up to frequencies of about 5 kHz.  This becomes difficult in
            a trans-impedance type of amplifier arrangement.  A very wide-band amplifier may be needed.

    3.    The transformer is a critical component that must satisfy competing requirements of low resistance, high
            inductance, maintenance of inductance at DC of 20 mA, and small size and cost.  Most off-the-shelf
            transformers are not satisfactory in this application.  We have been able to get custom transformers
            from Midcom [reference] that fit this application.

    4.    Equalization is generally necessary in the modem to mitigate the combined effects of the transformer and
            networks (current loops).  The equalization may need to vary  to account for the possibility of
            from 1 to 15 current loops.  Using practical values for other circuit elements, the combination of these
            elements with the transformer creates a high-pass filter with corner frequency at or close to 1 kHz
            (low end of HART band).

 

    Wireless HART

    It seems lately that just about every form of communication is becoming wireless or has wireless as an option.  And, undoubtedly, HART or HART-like information is now being transferred by wireless.  But this is the result of gateways to HART and the use of non-HART protocols.  A wireless version of HART Protocol does not exist and probably won’t ever exist.  The reason is that HART Field Instruments would have to be equipped with radio transmitters.  (Or there might be one transmitter serving several Field Instruments.)  This, in turn, implies a relatively large expenditure of power — much more than is currently used in HART Field Instruments.  And, since the power must be made available anyway, one might as well opt for an existing wireless protocol instead of creating a new one.  In other words, once we depart significantly from the conditions that led to the creation of HART in the first place, then other solutions become more viable.  This applies not only to wireless, but to any of the proposed alternate versions of HART described above.

    A market for transmission of process variables via wireless apparently exists.  But, based on information from potential customers, the requirement is for distances on the order of 15 miles (24 km).  This immediately excludes virtually all of the recently developed spread-spectrum unlicensed techniques, which are limited to about 1 or 2 miles of reliable transmission.

 


Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s