Hello, we have a 417-5H CPU with a 137-6AA00-0BA0 200SP CM Ptp module. The CM module is a Modbus RTU slave. We are using CFC in Step7 V5.5+SP4 (not TIA Portal). We are using blocks FB640 Modbus_Comm_Load V3.0 and FB642 Modbus_Slave V1.2 together with FB613 Send_P2p V1.2 and FB614 Receive_P2P V1.5, all from Modbus_RTU_V1.2 library.

I have a STL FB managing the Modbus slave communication, I am able to execute the Modbus_Comm_Load and get the DONE output active. When I try a read command (FC 3) with an external Modbus Master Simulator (Simply Modbus Mater), I only get a one flashing in the Rx LED on the CM module, and then an error code W#16#B100 on the Receive_P2P.Status. The Modbus_Slave.Status shows W#16#8280. And no response is received in the Modbus Master.

I can find the explanation of 8280 in the programming manual "ET 200MP/ET 200SP PtP communication instructions used with distributed I/O of an S7‑300/400", but it is not so helpful. Furthermore, there is no explanation for error code B100.

Any ideas on what is the issue or how to solve it?

I have attached a few pictures of my blocks/HW/DB for your reference.

Many thanks in advance!
Albert