Modbus RTU issue with Wago 750-8212 and PROFIBUS 750-960

CODESYS
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1

Hello, I’ve been developing in Codesys for a short time and I’m having a problem communicating with slave devices via Modbus RTU. To summarize my problem, I decided to build a small prototype of my project at home until I managed to solve the problem.

I have a Wago 750-8212 controller with firmware 28. To which I have connected a PROFIBUS 750-960 device for communication with the devices through a serial connection using Modbus RTU.

I’ve read every post I can about similar problems, many of which are from years ago, so many solutions are no longer possible.

I can summarize that the prototype I have at home, being small in scale, doesn’t present any wiring issues. I only have one device connected to communicate with Modbus, and it’s a Thermokon Novos 7. I’ve installed a 120-ohm resistor as the line termination. (I’ve tried other devices, but the error is the same, so I’ll only leave the device mentioned above connected).

This device is connected directly to my PROFIBUS 750-960 device which in turn is connected to my 750-8212 controller. I’m using Codesys 3.5 SP19 Patch 7.

The problem comes because when I try to read any record from my device it always throws me the error TIMEOUT RESPONSE

Of course I have checked my communication parameters (baud rate, address…) several times since if I connect with an external program called Modbus Master Emulator using a USB to RS485 adapter it can read all the registers and write without any problem. I also checked that my controller had serial communication assigned via RS485.

Despite everything, I’m still getting a timeout error. I must be making some basic mistake I haven’t noticed.

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Thank you in advance for any help you can give me.

2

In FW28, you should check the Web-Based Management for your Serial Port mode. Go to Administration → Serial Interface. The default is now RS232, but you can change it here to RS485.

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3

I’ve got it now. In the image I posted above left, you can see that I now have RS485. It’s one of the first configurations I changed. Thanks for responding. I’ve tried many solutions, but none have worked for me.

4

Sorry, I missed that and clearly see it now. The wiring and comm settings (baud rate, etc) are the most likely suspects, but it sounds like you have ruled those out as well.

5

Exactly. I’ve checked the wiring and configurations, and all the data is correct. It’s the same configuration I use to communicate with devices via Modbus Master Emulator. I’ve swapped cables and tried many things, and nothing seems to work.

6

Are you using the integrated termination of the 750-960 ?
Is the line polarized ?

7

Thank you very much. I’ve finally found the problem. Apparently, the resistors I was using for the end-of-line switch were defective. It wasn’t a Codesys or configuration issue. Thanks everyone.

1 Like
8

nice to know, can you flag this as resolved ?
Thanks