Is There Life After HART?
4800 BPS HART: A Proposal
Software and System Considerations
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FASTHART keeps most existing software. That is, assembling outgoing messages, checking for errors, parsing and interpreting incoming messages all stay the same. The major change is related to backward compatibility (see below) with existing HART. FASTHART devices must implement both FASTHART and existing HART. The modem chip must have an input signal that tells it which speed to use. FASTHART software must have the ability to set this input. The modem chip must also have an output signal that tells what kind of message is being received. New software must be able to read this output signal. The new software must also program the UART to either the 1200 BPS or 4800 BPS speeds. It must do this on-the-fly during receive, because the incoming message could be either speed.
Clearly, there
is the potential for system timing problems with a 4X increase in bit rate. For
example, a microcontroller may already be used to maximum capacity and be unable to handle
the higher data speed. Such problems may require additional software changes or may
preclude the use of the higher speed in some devices. The proposed method is only a
drop-in replacement in the sense that software continues to do the same things as before
and in the same sequence. A FASTHART product will be a new product, subject to
extensive testing to turn up these problems.
Backward compatibility and coexistence of HART and FASTHART are essential. An end-user must not have to worry about mixing devices on a network. Any device, old or new, should work on any network, old or new. In other words, an old Master should be able to query any field device at slow speed, including field devices capable of both speeds. A FASTHART Master should be able to query an older field device at slow speed. And, of course, a FASTHART Master must be able to query a newer field device at high speed.
This can happen only if the modem does both speeds. As explained above, additional modem pins control and indicate speed. A field device must answer at the same speed that it is queried. Older devices will see the high-speed messages as erroneous and will not act upon or respond to them.
High speed Masters can establish which Field Instruments are high-speed-capable through any of several means. One is through a DDL for the Field Instrument. Another is to use some reserved bit or flag in the Field Instrument response to command 0. In this case the Master would query the Field Instrument at slow speed to determine whether it is capable of high speed. Another way is to create a new Universal command that asks a device whether the Field Instrument is high speed capable. Again, a slow speed query is used to determine whether a Field Instrument is high-speed capable. Field Instruments that do not implement high speed would reply with "command not implemented" status.
A coexistence problem, not so easily solved, is that Masters must parse messages as part of arbitration. A combination of an older Master and a newer Master doesn't work because the older Master can't read the high speed transactions. Thus, a limitation of FASTHART may be that Masters can't be mixed. That is, if one of the Masters is high-speed capable, then the other must be. Or else there must be a way of turning the high speed capability OFF. It's probably reasonable to require end-users to avoid using incompatible Masters. But this is something that needs further examination..
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