Interfacing PLCs as Modbus slaves via serial to Ethernet converter


Thread Starter

Tom C.

I inherited a project from 2007 that is now in the startup phase. The field engineer apparently did not receive any interconnection information regarding two subsystems to the main control system. The Local Process Control system is a Quantum PLC (Schneider), the BMS portion is a HIMA, the two sub-systems are both Rockwell, one a MicroLogix 1100 and the second a MicroLogix 1500. We are using Schneider Serial to Ethernet converters (174-CEV by Schneider I think).

When a MicroLogix (either one) is configured as a Modbus Slave does that setup automatically convert the output to the Modbus RTU communications format? Can I connect directly from the Processor Channel to the RS-485 terminals of the converters?

Lynn August Linse

You might need to talk to Rockwell - I don't think older versions of the 1100 or 1500 support native Modbus. RSLogix 500 should be smart enough to know if your firmware is new enough to have Modbus support. (You mentioned this is 5 year old hardware ... )

A quick check of the 1100 User Manual says to look in the 1763-RM001 PDF, which is the Instruction Set Reference Manual. That probably explains how the RSLogix 500 MSG block works.