Problem with multiple ModbusTCP PLCs on a single IP Address.

[adambutt]

I am using a Modbus gateway to provide communications to multiple RS-485 Modbus PLCs through a single IP address. Each PLC has a unique Modbus Unit ID, and I can confirm Modbus communications to all slave devices simultaneously through the gateway using Modbus Poll and ModScan32.

In OA I set up a Modbus PLC for each Unit ID and using the same IP address for each. When I activate the PLC for each Unit ID individually, everything works. I can poll my registers and write my setpoints. However, if I enable more than one PLC, the communications for all units drops and the OA log starts filling up with the following errors:

WCCOAmod (1), 2015.06.25 13:52:34.810, PARAM,WARNING, 54, Unexpected state, ModModbusMaster, decodeFrame, response 1068 does not match to request 1069 for server 10.46.1.11
WCCOAmod (1), 2015.06.25 13:52:34.845, PARAM,WARNING, 54, Unexpected state, ModModbusMaster, decodeFrame, response 1069 does not match to request 1070 for server 10.46.1.11
WCCOAmod (1), 2015.06.25 13:52:35.855, PARAM,WARNING, 54, Unexpected state, ModModbusMaster, decodeFrame, response 1070 does not match to request 1071 for server 10.46.1.11

Does anyone have any experience using Modbus gateways with multiple PLCs, or have any ideas on what may be causing this? Thank you for your help.

[Gertjan van Schijndel]

How do you identify the different devices after the gateway? Are you using different unit IDs? Or different address/port combinations?