Example for slave

[yarosdev]

How to send request to the client?

I have following code:

listener.on('client', function(client){
    // how to send request?
});

const slave = new modbus.Slave({
    listener: listener,
    transport: transport,
    requestHandler: handleRequest
});

I'll be very thankful!

[morkai]

Option requestHandler must be specified and be a function(number unit, Request request, function(any) respond). For example:

function handleRequest(unit, request, respond)
{
  if (isValidRequest(unit, request))
  {
    respond(new Buffer([1, 2, 3, 4]));
  }
  else
  {
    respond(ExceptionCode.IllegalDataAddress);
  }
}

respond() takes a single argument that can be one of:

• ExceptionCode enum - will send an exception response
• an instance of Response corresponding to the received Request
• an instance of Error (more like an object with a message property) - will send the Slave Device Failure exception
• an instance of Buffer - will try to create an instance of Response corresponding to the received Request by using the Response.fromBuffer() (for example, if the request is ReadHoldingRegistersRequest, then the ReadHoldingRegistersResponse.fromBuffer() will be called)
• an object - will try to create an instance of Response corresponding to the received Request by using the Response.fromOptions().

See examples\slave.js for a full example.

[yarosdev]

What about transactions if it is tcp/ip?

[morkai]

Use TcpListener and IpTransport.

[yarosdev]

That's clear I mean executing concurrent transactions.

[morkai]

The respond() callback is unique to each request. If you receive 10 requests, the requestHandler will be called 10 times, each with different request and respond.

[yarosdev]

Thank you a lot! And last one question. Am I correct? To send command to the client I should send it in the request handler by calling a respond function?

[morkai]

Yes, you must call respond() to send a response to a request.

[yarosdev]

So, is modbus.Slave for emulating modbus device?

[morkai]

Yes, modbus.Slave=server and modbus.Master=client.