How can I set a custom slave ID for a Modbus TCP server using EasyModbusTCP in C#?

In Modbus TCP the concept of a "slave id" isn’t used in the same way as in serial Modbus (RTU/ASCII). Instead, the Unit Identifier field is available in the Modbus TCP packet to support scenarios such as communicating through gateways that connect to multiple serial slaves. In a typical TCP implementation, the server is identified by its IP address, and the Unit Identifier (which you can set via the UnitIdentifier property in EasyModbusTCP) is mainly useful for those serial-to-TCP gateway scenarios.

With EasyModbusTCP, when you set the UnitIdentifier on the client side, it is sent as part of the Modbus request. However, on the server side the library does not treat this field as a configurable “slave id” in the sense of having multiple logical endpoints within a single server application. The server simply listens on the configured IP address and port (typically port 502) and does not provide a built‐in mechanism to create multiple slaves distinguished by distinct id values in the 0–255 range.

If your requirement to have multiple slave ids in a TCP environment is driven by an application need (for example, mimicking a serial system behind a gateway), you have a couple of options:

1. Custom Routing Logic:
   You could modify the server-side code from EasyModbusTCP to inspect the Unit Identifier from incoming requests and then implement your own routing mechanism based on that value. This would involve extending the library’s functionality.

2. Multiple Server Instances:
   Depending on your network configuration, you could run multiple instances of the TCP server on different IP addresses (or ports) to mimic having distinct slaves.

In summary, EasyModbusTCP does allow setting a UnitIdentifier in requests, but the underlying Modbus TCP standard (and the library) does not support configuring a separate slave id on the server beyond that mechanism. The default behavior is to handle requests based on IP addressing rather than the 0–255 slave id range you might be familiar with in serial Modbus.

Below is an example of setting the Unit Identifier on the client side:

CSHARPCopied
using EasyModbus;
using System;

class Program
{
    static void Main(string[] args)
    {
        // Create a Modbus client connected to the modbus TCP server
        ModbusClient modbusClient = new ModbusClient("192.168.0.10", 502);
        
        // Set the UnitIdentifier (this is what is sent in the transaction)
        modbusClient.UnitIdentifier = 1;
        
        // Connect and perform your modbus operations...
        modbusClient.Connect();
        
        // Read some registers
        int[] readRegisters = modbusClient.ReadHoldingRegisters(0, 10);
        Console.WriteLine(string.Join(", ", readRegisters));
        
        modbusClient.Disconnect();
    }
}

For a server implementation with distinct slave id behavior, you would need to modify the library to check the Unit Identifier in incoming requests and then dispatch the request accordingly, as the standard EasyModbusTCP server does not provide direct support for multiple "slave" endpoints based on slave id.

Hope this helps clarify the situation!