Modbus RTU Arduino <-> PLC

Using Arduino Networking, Protocols, and Devices
1

Hello,

Does anyone have experience with Modubus RTU communication between Arduino and PLC?
I would need help communicating between an Arduino Nano Every and a Siemens S7-1200 PLC (specifically the 1212C AC/DC/Rly) using the Modbus RTU protocol. I am using a TTL to RS485 converter (MAX485) at Arduino side and RS485 communication board (CB1241) at PLC side.
PLC as Master and Arduino as Slave. I have a total of 4 slave devices. Communication on the PLC side is somehow solved with a simple script. I'm not so sure about Arduino. I tried the ArduinoModbus.h example Kitchen Sink library, which worked 100% between two arduinos. After the modification, when I want to light the LED on pin 13 after a command from the PLC using the Coil or the value in the Holding register, it no longer works so well.
I checked the flow through an oscilloscope and the data is in the correct format in both directions. However, Arduino "doesn't want to scramble" the data in any way.
I tried different libraries but with exactly the same result. I've been messing with it for over a week now.
After solving the problem, I will share the working code to future generations in trouble.

Thank you
Regards
Jenda

2

Works that well between 2 arduino or is that changed after testing with the arduino?

You can snif the RS485 in the arduino seriël monitor with an RS485=>USB converter.

3

Hi buckfast_beekeeper,

Communication works well between two arduinos.
I ordered RS485 to USB converter now and I will test it.
I'll inform about testing.

Thank you and best regards

4

That's not the best choice for AVR and MegaAVR Arduinos (as the Nano Every) because it's quite wasteful with memory. That library is designed for the MKR series of Arduinos.
I recommend the library by smarmengol for the Modbus RTU on AVR/MegaAVR boards.

Many PLC manufacturers use a special register numbering to select the register type. So often a register number 30000 selects holding register 0. I don't know what Siemens does but from this company I would at least expect it to be documented.

5

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