Any example for ExceptionResponse? about tokio-modbus

[LenRemmerswaal]

I would think that you could have the modbus server return an error response, by returning from call() like so:

 . . . 
    let fncode = tokio_modbus::coderc::req_to_fn_code(&req);
    return std::future::ready(Err(tokio_modbus::frame::ExceptionResponse {
        function: fncode,
        exception: tokio_modbus::frame::Exception::IllegalFunction,   // Or any other kind of Modbus exception
    }));
 . . .

Like mentioned before, for this to work, the following needs to be published:

• frame::ExceptionResponse
• frame::Exception
• codec::req_to_fn_code()

The first 2 could be included in the prelude, like Request and Response are.
The last 1 I am not sure about.

If you return with another kind of error than an ExceptionResponse, like std::io::Error, the application compiles just fine, but the connection is forcibly closed by the server when the error is returned.

The workaround from @justinyhuang (clever!) works fine:

        return std::future::ready(error_response(&req, Exception::IllegalFunction));

Copy the following functions and enum to make this work:

fn error_response(req: &Request, exc: Exception) -> std::io::Result<Response> {
    Ok(Response::Custom(req2functioncode(&req) | 0x80, vec![exc as u8]))
}

fn req2functioncode(req: &Request) -> FunctionCode {
    match *req {
        Request::ReadCoils(_, _) => 0x01,
        Request::ReadDiscreteInputs(_, _) => 0x02,
        Request::WriteSingleCoil(_, _) => 0x05,
        Request::WriteMultipleCoils(_, _) => 0x0F,
        Request::ReadInputRegisters(_, _) => 0x04,
        Request::ReadHoldingRegisters(_, _) => 0x03,
        Request::WriteSingleRegister(_, _) => 0x06,
        Request::WriteMultipleRegisters(_, _) => 0x10,
        Request::ReadWriteMultipleRegisters(_, _, _, _) => 0x17,
        Request::Custom(code, _) => code,
        _ => 0x00,
    }
}

#[repr(u8)]
enum Exception {
    IllegalFunction = 0x01,
    IllegalDataAddress = 0x02,
    IllegalDataValue = 0x03,
    ServerDeviceFailure = 0x04,
    Acknowledge = 0x05,
    ServerDeviceBusy = 0x06,
    MemoryParityError = 0x08,
    GatewayPathUnavailable = 0x0A,
    GatewayTargetDevice = 0x0B,
}

[justinyhuang]

Any response to this question? it appears to me that the exception definitions are in frame which is private.
How could a server implementation produce an appropriate exception response in this case?

For what is worth, below is my workaround:
the client side appears to be taken care of so if you are only interested in building a client the current implementation would work just fine.
the server side however does not get support from this crate at the moment, the way I get it working is to overload the custom response type to send back the exception response. though it is not clean and could be confusing, but it works for now.

[glennpierce]

I wanted to match on

Custom { kind: Other, error: ExceptionResponse { function: 3, exception: GatewayTargetDevice } } => {

}

but you are right frame is private where I find myself needing a few things from it.
In the end I converted the error to a string and did a match (yuck)