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:

1. Connected pins 3 and 8 from the DB9 connector to the +/- RX/TX terminals on the Carel side.
2. In the PLC shell, I set serialmode to RS485.
3. In WBM under Administration > Serial Interface > Assign owner of serial interface, I set the port to unassigned.

4. In the device tree, I created Modbus_COM with the following settings:

5. COM port: 1

6. Baud rate: 19200
7. Parity: None
8. Stop bits: 1
9. Created a Modbus client on this COM interface in RTU mode. Under the status tab, I can see it is in running state.
10. On the client, I created a Modbus server and set the Modbus address of my unit to 1.
11. 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?