CompactLogix RS-485 Modbus master question

What would I need to communicate as a Modbus RTU master device via RS-485, from a CompactLogix 1769? The only choices I can find is the built in RS-232 port or the 1769-ASCII, which may not work with Modbus RTU.

There are several ways to perform Modbus RTU Master on RS-485 from a CompactLogix controller, and they follow the usual linear complexity vs. expense equation.

1. CompactLogix serial port, Modbus RTU ladder code (from Samplecode website) with 1761-NET-AIC or similar RS232/RS485 isolator.


2. 22-SCM-232 DF1/Modbus RTU converter attached to CompactLogix serial port, with DF1 MSG instructions.


3. DeviceNet -> Modbus RTU or EtherNet/IP-> Modbus RTU converter box from Prosoft, HMS Fieldbus, Realtime Automation, Western Reserve Controls, etc.

4. MVI69-MCM Modbus RTU module from Prosoft.

If you are building just one unit and are inexperienced with Rockwell gear, the Prosoft MVI69-MCM module is the easiest to set up and the easiest to troubleshoot.

Ken Roach said:

3. DeviceNet -> Modbus RTU or EtherNet/IP-> Modbus RTU converter box from Prosoft, HMS Fieldbus, Realtime Automation, Western Reserve Controls, etc.

Click to expand...

I hadn't thought of going the EtherNet/IP to converter route, that is a good one. I have a lot of experience using FieldServers and can get them for a good price. Thanks!