RTU Custom Request Mismatching Function Code

[EzekielDaun]

Thanks for this awesome crate!

I can use read_holding_registers without issue. However, when I send a custom request with function code 0x03 (to mimic read_holding_registers), the call

let request = tokio_modbus::Request::Custom(3, Cow::Owned([0x01, 0x52, 0x00, 0x01].to_vec()));
ctx.call(request).await??;

returns:

Error: mismatching function codes: 3 Ok(ReadHoldingRegisters([47826]))

I skimmed through the source code but still couldn’t figure out where this mismatch is coming from. And the value 47826 is correct.

Am I misunderstanding how the custom request is supposed to be used?

[flosse]

Are you sure that the server also sends back 0x03?

The check is generally done here:

tokio-modbus/src/service/rtu.rs

Lines 84 to 94 in 4a061c4

	let rsp_function_code = match &result {
	Ok(response) => response.function_code(),
	Err(ExceptionResponse { function, .. }) => *function,
	};
	if req_function_code != rsp_function_code {
	return Err(ProtocolError::FunctionCodeMismatch {
	request: req_function_code,
	result,
	}
	.into());
	}