https://forge.codesys.com/forge/talk/Runtime/thread/e81855c1d7/2026-05-22T09:36:11.486000ZRecent posts to PFC200 - WAGO 750-8202 - MODBUS RTU 2026-05-22T09:36:11.486000Z2026-05-22T09:36:11.486000Zolehmann03https://forge.codesys.com/u/olehmann03/https://forge.codesys.com7a791f9fd1faa5a1c73f3ce8897b15905a8f5c99

<div class="markdown_content"><p>Hello,</p> <p>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.</p> <p>Below are the steps I’ve done so far:</p> <ol> <li>Connected pins 3 and 8 from the DB9 connector to the +/- RX/TX terminals on the Carel side.</li> <li>In the PLC shell, I set <code>serialmode</code> to <code>RS485</code>.</li> <li>In WBM under <code>Administration &gt; Serial Interface &gt; Assign owner of serial interface</code>, I set the port to <code>unassigned</code>.</li> <li> <p>In the device tree, I created <code>Modbus_COM</code> with the following settings:</p> </li> <li> <p>COM port: 1</p> </li> <li>Baud rate: 19200</li> <li>Parity: None</li> <li>Stop bits: 1</li> <li>Created a Modbus client on this COM interface in RTU mode. Under the status tab, I can see it is in <code>running</code> state.</li> <li>On the client, I created a Modbus server and set the Modbus address of my unit to <code>1</code>.</li> <li>Defined a few channels as FC03 to read holding registers according to the Carel register map.</li> </ol> <p>I checked the communication with the Carel controller using a USB-to-RS485 converter connected to my PC, and communication works correctly there.</p> <p>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.</p> <p>Do you have any suggestions on what else I should check?</p></div>