PFC200 - WAGO 750-8202 - MODBUS RTU

olehmann03 — Fri, 22 May 2026 09:35:24 -0000

Hello,

I’m working on implementing Modbus RTU communication with a Carel controller (CPCO). I want to use the front RS485 interface through the DB9 connector. The issue is that I can’t get the communication working correctly.

Below are the steps I’ve done so far:

Connected pins 3 and 8 from the DB9 connector to the +/- RX/TX terminals on the Carel side.
In the PLC shell, I set serialmode to RS485.
In WBM under Administration > Serial Interface > Assign owner of serial interface, I set the port to unassigned.
In the device tree, I created Modbus_COM with the following settings:
COM port: 1
Baud rate: 19200
Parity: None
Stop bits: 1
Created a Modbus client on this COM interface in RTU mode. Under the status tab, I can see it is in running state.
On the client, I created a Modbus server and set the Modbus address of my unit to 1.
Defined a few channels as FC03 to read holding registers according to the Carel register map.

I checked the communication with the Carel controller using a USB-to-RS485 converter connected to my PC, and communication works correctly there.

In the status tab, I can see that the Modbus server is polling, but there is no response from the controller. I also tried multiple times swapping A/B lines on the Carel side, but it did not help.

Do you have any suggestions on what else I should check?

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