Compressors and Modbus Communcations

We were unable to read data continuously from all the three compressors at a time.

I hope that this is just a miscommunication. Modbus is a sequential polling protocol.

In my experience, RS485 is the most commonly used electrical method of communicating to devices in an industrial plant, when using the Modbus protocol. Things to watch out for: Make sure you have a daisy-chain connection (Modbus master to unit 1, unit 1 to unit 2, unit 2 to unit 3...); check to make sure that the terminating resistors are installed on both ends, and not in the middle.

Hope this helps.