Modbus is a communications protocol based on master/slave or client/server architecture, designed by Modiconfor use with its PLCs. It has become a standard wherever Industrial Automation Systems (IAS) or Building Management Systems (BMS) needed. The main reason for the extensive use of Modbus over other communications protocols is the fact that it is openly published, its relatively easy to implement and reliable.
Modbus comes in a variety of flavors, RTU and ASCII. Both of these procols are serial (RS232 or RS485) based.
Modbus RTU is a compact, binary representation of the data.