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Binary Read Request - Ambigious responses for Memory Bit read request

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Hi, I hope you might help with the following issue:

I wrote  a simple python library, using the PCOM protcol &  pySerial. To test it,  I created a script that reads a few Memory Bit operands from a SM-43 via the built-in RS232-USB Bridge. The library and the script work perfectly, accept when it comes to reading more than seven bit operands:  I used the online mode to set the MB100-107 to the values : 10101100.  While the response comes back intact, ( checksum checks out etc) I'm getting different responses  for the same request.  The respective bytes in the payload of the response message vary between:

  • 0x3521
  • 0x3523
  • 0x3544
  • 0x3543
  • 0x35cb

Note that

  1.  I made sure that the actual bit values in MB100-107 do not change
  2. The response is consistent, as long as I don't change the status of MB107. However, as soon as I'm toggling MB107 on and off again, i get a different response from the PLC afterwards. I also get a different response when I change the read request and then go back to the old read request.
  3.  The first seven bits do not seem to be affected by this - They show 0x35 consistently.

Maybe I overlooked something?  Thanks in advance for any hint!




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