Slave ID Importance in MODBUS TCP/IP


Thread Starter


I'm communicating my LnT Relays to a SCADA system through a DC over Modbus TCP/IP protocol. On the DCU, I have 2-3 controllers.

My vendor who says there is no significance for device ID in MODBUS TCP/IP, has used a common device id for all 2-3 controllers. Now, I'm facing a problems of the second controller not being able to communicate continuously. My SCADA controller fails frequently as the slave response is getting interrupted very frequently.

IS this problem because of the wrong device IDs?
Don't think so. Everything I have read about Modbus TCP is that the UID is not used. A few days ago I checked with a Schneider M258 PLC and it didn't care too.
Modbus TCP devices are traditionally individualized by their IP address in Modbus TCP networks but you will still need to know the Modbus ID for the device since it is required by the protocol for communication.

The Scada package requires an IP Address, port number (usually 502) and the Modbus ID (node number) for creating the connection to the device. At least one of these must be distinct to identify the device. Usually, it is the IP address in Modbus TCP networks but all are required to know to set up the connection.

We have webinars showing how to connect using SCADA software and about Modbus communicaiton that can be considered as a tutorial which may help you to understand how to connect in your SCADA software.
Our schedule and archived trainings can be found here: