Slave Id. Add the slave here by entering a slave identifier in the text field, and then press Add. (The new slave appears as a tab labelled with the slave id in the main window.) The Slave Id is what is called the "slave address" in the original Modbus specification, and is called the "unit identifier" in the Modbus/TCP specification. It should be in the range 1 to 255. A zero slave identifier is used in write requests as a broadcast identifier (each slave that receives the message should store the values supplied in the request, but should not send a reply). The original Modbus specification limited the range of slave identifiers to 1 to 247, for no apparent reason. The Modbus/TCP implementation guide suggests that Modbus/TCP slaves that can be uniquely identified by their IP address should use slave identifier 255. You may not add a slave when ModSlaveSim is serving the master or running the slave simulation (i.e. when Serve Masters or Run Slave Simulation are highlighted).