Modbus

Modbus is a serial communication protocol developed by Modicon in 1979 for use with their programmable logic controllers (PLCs).
Communication is carried out using a master/slave method, where the slave responds and acts according to commands from the master.
Modbus is defined only as a communication protocol, while the physical transmission medium most commonly used includes RS-232C, RS-485, and Ethernet.
There are two transmission modes: “Modbus ASCII mode,” which converts one byte (8 bits) of data to two ASCII characters for transmission, and “Modbus RTU mode,” which transmits one byte (8 bits) of data as is.