modbus server implementation on raspberry to enable/disable GPIO pins via serial modbus client

[J0ergL]

Dearest community.

facing the challenge of implementing the topic I thought this should not be to hard.
Investigating on the topic proofed me otherwise. My searches on the internet turn up examples, bound to different versions to pymodbus and therefore not running on version 3,11.1 using python 3.11.

I am working my way thought the examples of pymodbus, in particular server_*.py now and I am wondering if anyone here in the community was facing the same challenge and is willing to share the code with me or write suggestions on how to solve it.
Thank you very much

[janiversen]

I think you should rather be looking at pyserial, since Pymodbus do not handle the lower serial communication.

The lowest level in Pymodbus serialtransport.py but that merely handles addition of asyncio.

Since Pymodbus server is totally asynchronous you probably will find it rather difficult to sync the gpio with the actual transmission of data.

The way Pymodbus interfaces with pyserial have not changed over the last couple of years, so you should be able to look at old examples.

[J0ergL]

@janiversen Thank you for the fast reply and your suggestion!
What problems with lower serial communication are you thinking of?

"Pymodbus server is totally asynchronous"
I am under the impression:
In the event of a register or a coil (do know which approach would be better) is rewritten by a client, the server can react on the change of that register or coil accordingly meaning setting the state of a GPIO pin.
Maybe my understanding of the whole pymodbus play is incorrect or maybe I drew the wrong picture, of what I am trying to accomplish.
I want to be able to set the state of a GPIO via modbus RTU on a raspberry.

I am looking into pyserial and so far I don't see how this will help me.
Where as the pymodbus server_*.py examples feel like useful pointers on how to accomplish to set the state of a GPIO via modbus RTU on a raspberry.

[janiversen]

I thought your intention was to use the gpio to control the communication. The title says "serial" and enable/disable gpio, so it makes sense to think it has something to do with serial communication.

If you just was set/unset a pin when the state of a coil changes, independent of the type of server, there are at least one example showing to make a custom datastore.

You can also use the trace points, which might be easier, by attaching a function that does what you want.

[janiversen]

"I want to be able to set the state of a GPIO via modbus RTU on a raspberry." I am not sure I understand what you think of ??? Pymodbus have servers and clients...."modbus RTU" is normally a physical device. So are you having a modbus device connected to your raspberry ?? In this case the raspberry would use the Pymodbus client not the server (which simulates a modbus device).

[J0ergL]

I thought your intention was to use the gpio to control the communication. The title says "serial" and enable/disable gpio, so it makes sense to think it has something to do with serial communication.

If you just was set/unset a pin when the state of a coil changes, independent of the type of server, there are at least one example showing to make a custom datastore.

You can also use the trace points, which might be easier, by attaching a function that does what you want.

I see I have to change the title. And I'll check "trace points".

[J0ergL]

"I want to be able to set the state of a GPIO via modbus RTU on a raspberry." I am not sure I understand what you think of ??? Pymodbus have servers and clients...."modbus RTU" is normally a physical device. So are you having a modbus device connected to your raspberry ?? In this case the raspberry would use the Pymodbus client not the server (which simulates a modbus device).

The raspberry has an USB to RS232/485 device connected.
I want to implement a modbus server, that runs on the raspberry. That modbus server shall support to set the state of GPIO pins of the raspberry with a modbus client.