Is the Module Index Equivalent to Modbus Slave Id?


Thread Starter



One of our customers who is using our Modbus library "Modbus Modern API - Java" had mentioned that he has a device which supports Modbus TCP. The device has got three modules. To poll a particular register address, the vendor had asked my customer to use slave Id 3 in the Modbus request. My client had confirmed that it works fine with slave Id 3.

Is module index 3 an industry standard or its vendor specific?



Hi Wasan,

Thanks for your response. Yes, I know that Modbus TCP needs IpAddress. Our library support both TCP and UDP.
Maybe I describe unclearly for your question.

"Is module index 3 an industry standard or its vendor specific?"

It is vendor specific. (Most devices use slave ID 1 but some use other ID).

Fred Loveless

The simple answer is yes you can treat them the same. There are a lot of devices out there that started out as Serial devices and the manufacturers have extended their life by adding a Modbus TCP to Modbus RTU gateway onto them. I have seen devices with multiple internal indexes to pass requests to different modules or IO segments on those devices. Typically as stated before the default of those devices is 1.