InvalidResponseError: Wrong functioncode

[ollie-campbell]

Hi, I'm using minimalmodbus (2.0.2) to read data from a system connected to a Raspberry Pi over USB-to-serial convertor and it's working well. I would like to try and write data to it now, but am running into a problem.

When I read a register I get the following:

instrument.read_register(192) MinimalModbus debug mode. Will write to instrument (expecting 7 bytes back): C9 03 00 C0 00 01 94 7E (8 bytes) MinimalModbus debug mode. Clearing serial buffers for port /dev/serial0 MinimalModbus debug mode. No sleep required before write. Time since previous read: 127454.06 ms, minimum silent period: 4.01 ms. MinimalModbus debug mode. Response from instrument: C9 03 02 00 04 58 57 (7 bytes), roundtrip time: 0.1 ms. Timeout for reading: 150.0 ms.

4

If I try to write value "0" to this register I get the following:

instrument.write_register(192, 0) MinimalModbus debug mode. Will write to instrument (expecting 8 bytes back): C9 10 00 C0 00 01 02 00 00 80 95 (11 bytes) MinimalModbus debug mode. Clearing serial buffers for port /dev/serial0 MinimalModbus debug mode. No sleep required before write. Time since previous read: 7018.76 ms, minimum silent period: 4.01 ms. MinimalModbus debug mode. Response from instrument: C9 01 00 A0 6E (5 bytes), roundtrip time: 0.2 ms. Timeout for reading: 150.0 ms.

Traceback (most recent call last): File "", line 1, in File "/home/pi/.local/lib/python3.9/site-packages/minimalmodbus.py", line 550, in write_register File "/home/pi/.local/lib/python3.9/site-packages/minimalmodbus.py", line 1245, in _generic_command payload_from_slave = self._perform_command(functioncode, payload_to_slave) File "/home/pi/.local/lib/python3.9/site-packages/minimalmodbus.py", line 1329, in _perform_command payload_from_slave = _extract_payload( File "/home/pi/.local/lib/python3.9/site-packages/minimalmodbus.py", line 1885, in _extract_payload raise InvalidResponseError( minimalmodbus.InvalidResponseError: Wrong functioncode: 1 instead of 16. The response is: 'É\x01\x00\xa0n'

Am I reading it correctly that the unit I am querying is responding from a different function code and minimalmodbus doesn't like it?

[j123b567]

minimalmodbus 2.0.2 does not exists (latest is 2.0.1), do you have some custom modifications?

Am I reading it correctly that the unit I am querying is responding from a different function code and minimalmodbus doesn't like it?

Yes

If the request is C9 10 00 C0 00 01 02 00 00 80 95 then the response should be C9 10 00 C0 00 01 11 BD but your device responded with C9 01 00 A0 6E (wrong function code, wrong response length)

[ollie-campbell]

Apologies 2.0.1, a mistype.

Ok thank you. I realise this isn't a bug issue now, but are you able to point me in a diagnosis direction? Something I can fire at the manufacturer for additional help.

[j123b567]

Maybe the Modbus protocol specification https://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf

Chapter 6.12 16 (0x10) Write Multiple registers

[ollie-campbell]

Thank you for your help so far. I have had a response from the manufacturers, who have given me Hex and binary to write to the device. Can I use this directly using minimalmodbus? Or is there a way I can 'convert' this to use it?

Examples from them are:

201 6 10753 78 C9 06 2A 01 00 4E 406E

Thanks again.

[j123b567]

In your example, the function used is 06 Write Single Register

• You can try instrument.write_register(10753, 78, functioncode=6)
• You can simply try to patch minimalmodbus in your project to ignore invalid responses generated by your instrument.

[ollie-campbell]

Great thank you once again.