Modbus RTU / RS485 Communication Issues with WAGO 750-8112 CPU

Hardware and Linux
1

Dear WAGO Technical Support Team,

I am experiencing communication problems with Modbus RTU over RS485 using a WAGO 750-8112 CPU, and I would appreciate your technical assistance.

The main issue is that communication with the same Modbus RTU devices works reliably when I use Modbus Poll, but I experience communication errors and timeouts when I communicate with the devices through the WAGO 750-8112 CPU.

Here is a summary of the situation:

  • WAGO CPU: 750-8112

  • Communication protocol: Modbus RTU

  • Physical interface: RS485

  • Modbus master: WAGO PLC

  • Devices: Several Modbus RTU devices, including ABB softstarters

  • The RS485 wiring and physical connections have been checked.

  • I have also installed the appropriate 120 Ω termination resistor, but the problem still occurs.

  • When I test the same RS485 network using Modbus Poll, the devices respond correctly and communication is stable.

  • The problem is particularly noticeable when communicating with ABB softstarters.

  • With some devices, communication works initially, but after adding more devices to the RS485 network, some devices cannot be read reliably.

  • Individual device testing can work correctly, while communication with multiple devices on the same RS485 bus becomes problematic.

Because Modbus Poll communicates successfully over the same physical network, I suspect that the issue may be related to the WAGO Modbus RTU configuration, timing, polling interval, response timeout, inter-frame delay, or another parameter in the WAGO Modbus master implementation.

Could you please advise me on the recommended Modbus RTU configuration for the 750-8112 CPU?

In particular, I would like to know:

  1. What are the recommended Modbus RTU timing parameters for the 750-8112?

  2. Is there a recommended minimum delay between Modbus requests?

  3. How should the response timeout be configured?

  4. Is there any specific configuration required when communicating with ABB softstarters?

  5. Are there any known compatibility issues between the WAGO 750-8112 Modbus RTU master and ABB Modbus RTU devices?

  6. Is there any recommended RS485 biasing/failsafe configuration for this CPU?

  7. Are there any specific recommendations regarding the maximum number of devices or cable length on the RS485 network?

  8. Are there any diagnostic or communication statistics available in the WAGO controller that can help identify whether the problem is caused by CRC errors, timeouts, or framing errors?

I can provide the complete Modbus configuration, PLC program, device addresses, baud rate, parity settings, RS485 wiring diagram, and communication error details if required.

I would appreciate it if you could review this issue and recommend the correct configuration or troubleshooting procedure.

Thank you for your support.

Best regards,

2

Hello. Does the communication also work when you connect the RS485 to the pc but without having the pc as master ? Maybe there is a bias missing somewhere.

3

Yes, it works perfectly on the computer. I think the RS485 procedures should be reviewed.

4

I was meaning: connect the computer and the pfc but only have the pfc running as master and don’t start the computer modbus master (let it on the disconnected mode).
To see if the hardware need something like the PC interface (which probably have the 5v and 0v bias resistor).
On the pfc what connector are you using ?

5

Extract of the manual on the serial port and rs-485 wiring:

IMG_2550
IMG_25501170×1062 140 KB

IMG_2549
IMG_25491170×718 59.9 KB

Most of the time it is because people don’t connect the bias resistor (the 450..650Ohm one)

6

I’m using a D-Sub 9 connector. I’m only connecting to pins 3 and 8.

7

Do we have to use these resistors in every application?

8

It depend. If you measure by a volt meter between your 3 and 8 you should have 5v. Next between your 3 and gnd, you should have 5v and between your 8 and find you should have 0. Only one set of resistor is needed.
If you use a connector like the 750-960 (if not mistaken) you can just switch the term resistor to also have the bias connected.
By a rule of thumb, device with a sub-D and no terminaison resistor don’t have those bias resistor too.
But yes it’s a good idea to have those on most rs-485 network is not there …

9

I don’t experience any problems with the RS485 line using different brands of PLCs without connecting any resistors. However, I always have problems with the RS485 when using a Wago CPU. I believe the RS485 port is very sensitive.

10

I would say “maybe” ?

I think the main problem came from the HW itself, since the port can be used as RS232 and RS485, the bias resistor can’t be put on the HW side. (voltage difference between the 2 HW is too important).
For this reason those aren’t here and should be added externaly.