ABSTRACT
The MODBUS serial communication protocol is based on master-slave principle. It is an application layer messaging protocol, operating at layer 7 of open systems interconnection (OSI) protocol. Layers 3–6 of OSI are not used here. Different MODBUS protocols viz., MODBUS on MB+, MODBUS on RS-232, MODBUS on RS-485, initiate remote communication by using Transmission Control Protocol (TCP)/Internet Protocol (IP). MODBUS provides for one master with 247 slaves. Devices such as programmable logic controllers (PLC), human machine interface (HMI), input/output (I/O), etc. are connected to the MODBUS TCP/IP via individual gateways.
Over a MODBUS network, two types of transmission of serial data are possible: remote terminal unit (RTU) and ASCII. The two modes differ in a number of ways: the manner in which information is packed in the message field, the way the bit contents of the message are interpreted, the way the message is decoded, and the speed of operation at a given baud rate.
A MODBUS PLUS network can support a maximum of 64 devices. On a single section maximum 32 devices (nodes) can be connected. Repeaters are used to join different sections. Such networks can also be used with dual cabling using repeaters.
