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Is the Module Index Equivalent to Modbus Slave Id?
In Modbus TCP is the module index = Modbus slave id?


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?


For Modbus TCP important topic is IP Address that point which Module you connect. But for some reason (e.g. Modbus Gateway), they have to assigned address to match slave id on device to communicte depend on each vendor to use any address. (Almost are 1)

Reference for below manual shown Modbus Slave Address can be 1 to 247

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).

By Fred Loveless on 10 September, 2018 - 11:58 am

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.

Thanks a lot to both Wasan and Fred.