Communication Arduino Mega to PLC using Modbus RTU RS485

[StergiosT]

Hello,

I am looking for a way to communicate the Arduino Mega as Slave and a PLC as a Master over RS232 or RS485.

Any suggestion is appreciated.

Regards.

[markd833]

For the hardware I would get yourself a 5V RS485 module (MAX485 chip). Something like one of these:

As for code, I've tinkered with this library out of curiosity a while back:

GitHub - yaacov/ArduinoModbusSlave: Modbus slave library for Arduino

If you are experimenting, then I would also suggest getting hold of an RS485-USB dongle like this one:

I would avoid the ones that only have the 2 pin connector that breaks out just A & B as they don't have a GND connection. The dongle can be extremely useful when debugging Modbus (or any RS485) comms as it will let you see what data is actually on the RS485 bus.

[StergiosT]

Thanks, finally i figured out using this cable rs485 usb cable

I succeed communication between arduino max485 and modbus slave software that runs in windows. Next stet is direclty to a PLC.
Is there any compatibility issue with brands like siemens, allen bradley, idec etc?
Or doesn't matter?

Thank you for any help you can provide.

[markd833]

@StergiosT

Is there any compatibility issue with brands like siemens, allen bradley, idec etc?

I'm not a PLC expert or even novice, but if it supports Modbus then it should adhere to the Modbus specs. I would have thought that the Modbus world would be designed with interoperability as standard, but maybe there could be some quirks.

[StergiosT]

Finally i figured out. Thank you!