Using a mixture of standard and custom response/request classes in a synchronous serial client

[pfsysadmin]

@janiversen , thank you for the pymodbus package.

I'm new to Modbus (and python) so it's taking me some time to learn them.

I have a requirement to interface with a particular measuring instrument via Modbus over an RS-485 connection.

I have determined by monitoring the serial traffic that the instrument uses a mixture of standard and custom Modbus function code requests and responses. In order to cope with this I thought I would create custom response and request classes based on the standard pymodbus ModbusResponse and ModbusRequest classes and registering the custom response class using the .register() function.

I was wondering it is possible to use both standard and custom function responses/requests in the same program?

From my investigations so far, the custom one works but when I then call the standard Modbus read_holding_registers(), I see from the debug logs it is using the custom response class (Factory Response[GetConfigCustomModbusResponse': 3]) instead of Factory Response[ReadHoldingRegistersResponse': 3].

If I don't do the .register(GetConfigCustomModbusResponse), the standard _read_holding_registers function works for the particular function code 0x3 request.

I have probably not done the sub class creation from the base class properly or made some other mistake(s). Sub class code follows.
class GetConfigCustomModbusResponse(ModbusResponse):
function_code = 3
_rtu_frame_size = 15
_rtu_byte_count_pos = 3

def __init__(self, values=None, slave_id=None, **kwargs):
    super().__init__(self, **kwargs)
    self.values = values or []
    self.slave_id = slave_id

followed by the various encode decode, etc def's.

There is only one .register(GetConfigCustomMopdbusResponse) call.

I look forward to comments, etc,

Regards

Paul

[janiversen]

You can have multiple custom requests, but if the function code is the same as a standard one, you overwrite the standard !

Same goes for responses, if you define an existing function code, the standard function is no longer available.

To send a custom request you need to use client.execute

[pfsysadmin]

@janiversen,
Thank you for your prompt reply and explanation. I am using client.execute.

Unfortunately the instrument has three function code 0x3 formats. One is the "standard"Modbus type while the other two are custom ones. The differences are basically about the byte count ( 2 bytes not one) and frame size.

Is there a way to support these three function code 0x3 types?

Thanks

Paul

You can have multiple custom requests, but if the function code is the same as a standard one, you overwrite the standard !

Same goes for responses, if you define an existing function code, the standard function is no longer available.

To send a custom request you need to use client.execute

[janiversen]

Yes make a custom class with 0x03 and in decode/encode methods call the standard one if it is a standard request/response

[janiversen]

However it is very rare to have 3 different 0x03 formats, because there are no sub-codes, how do you know which is which, not only to decode/encode the request but also to calculate the frame length.

[pfsysadmin]

@janiversen,
Thank you for your suggestions and comments in your last two replies.

I was wondering if you could give me some pointers about how to go about setting up the respective decode/encode methods in the custom classes of the ModbusResponse and ModbusRequest classes?

As to your last reply, I control what function code 0x3 request is sent so I know what response to expect. I could pass another parameter to the custom classes created from ModbusResponse/ModbusRequest classes, which indicated the type of request to send.

Is it possible to "un-register" the custom class? Or would you create an exact copy of the ModbusResponse class and register that?

Thanks again for you assistance. I greatly appreciate it.

Paul

[janiversen]

Passing another parameter will not help you with the responses.

You cannot unregister a custom class, it does not make sense, but of course you can stop the special handling in your code.

For pointers have a look at our examples.