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Is it possible to terminate the carrier signal at the zero crossing  using a Maxim DS8500 HART modem?

Posted on 2011-03-04
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Is it possible to terminate the carrier signal at the zero crossing  using a Maxim DS8500 HART modem?
It appears that if this is not done, the glitch at the end of the carrier gets interpreted as extra 0xFF’s which causes a communication error to occur.  This is required for HART  test certification. Please see attached waveform. The RTS (request to send) line is the yellow trace, the carrier is in purple, and the RS-232 received data on the test modem is in blue. The glitch is located at the very end of the carrier waveform. Maxim Modem waveshape
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Question by:farcuri
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Dave Baldwin earned 668 total points
ID: 35042038
I looked at the data sheets for a couple of HART modem ICs and none of the including the Maxim DS8500 showed the circumstance you're showing above.  None mentioned zero-crossings.  What happens if you wait a little longer to change the state of RTS?
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by:HappyCactus
HappyCactus earned 668 total points
ID: 35044173
could it be a cross-talking problem? Maybe with a different layout you can avoid the RTS to cause the bouncing of the analog signal?

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by:Dr. Klahn
Dr. Klahn earned 664 total points
ID: 35045164
Well, the modem signal is certainly cut right when RTS changes state.  Curious; I would have expected a modem chip to behave better, and delay until the next zero crossing specifically to prevent transients.

I suppose it's possible to use RTS as enable/disable on a zero-crossing switch such as the Fairchild ZCS chips.  It's doubtful that those chips were designed with variable frequency/variable phase signals in mind, so whether there will be distortion introduced by the ZCS is another matter.
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by:farcuri
ID: 35071602
The issue here does not appear to be crosstalk, what seems to be happening is that when the RTS line changes state, the DS8500 is supposed to terminate the signal and set the output to a nominal 1.25V.  This effectively releases the line so that HART masters can interrogate the slave device. What we have noticed is that the modem does not appear to monitor the zero crossings before it terminates, so there is a transient generated in the carrier that is interpreted as additional "0xFFs" or preambles that represent the start of a new message frame.  This violates HART requirements and is discouraged. We have tried using an analog switch that would force the output to the reference voltage once the RTS line transitions. The problem with this is that our carrier does not always terminate at the zero crossing. We have tried delaying the transition of RTS by sending additonal dribble bits at the end of the message, but this does not seem to fix the problem either. Does anyone have experience with HART qualification tests as this may shed some light on this subject.   Thank you.
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Expert Comment

by:Shayne943
ID: 37657363
We tried low pass filtering the output of the modem with no avail.  We also tried delaying the RTS signal with no avail.
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