Modbus address on RS-485

Hi all,

I have 15 devices on the rs-485 link which implements modbus RTU protocol. According Modbus RTU protocol, slave address ranges from 1-127. but rs-485 only allows 32 nodes, some device's address can only be set from 1 to 31. Does it mean I can not set address from 33-127 for any RS-485 modbus RTU device? Thanks,

Its not modbus that limits it to 0-31, its RS485 spec. that limit it to 0-31 node addresses. Modbus can go from 1-254.

You can get extenders to increase that nodes address limit. See below for one such device, there are many other vendors.

http://www.bomara.com/Telebyte/catalog/products/8321.htm

Thanks, What I mean is that can I set device address bigger than 31(for example: 50) on rs-485 modbus RTU network?

Mickey said:

Its not modbus that limits it to 0-31, its RS485 spec. that limit it to 0-31 node addresses. Modbus can go from 1-254. You can get repeaters for your RS485 network that will allow more nodes.

Click to expand...

Yes. The Modbus Slave Address has nothing to do with how many devices are connected to the RS-485 network.

Mickey said:

Its not modbus that limits it to 0-31, its RS485 spec. that limit it to 0-31 node addresses. Modbus can go from 1-254.

You can get extenders to increase that nodes address limit. See below for one such device, there are many other vendors.

http://www.bomara.com/Telebyte/catalog/products/8321.htm

Click to expand...

The RS-485 spec limit is 32 nodes. It doesn't care what address they are. The limitation is due to bus drivers not being able to drive more than 32 nodes.