Section 2
Modbus overview
2.1
Modbus standard
Modbus is a communication protocol developed by the Modicon company in the
1970’s. Originally it was used for communication in PLCs and RTU devices. Later on
the Modbus protocol has been used in a variety of different device applications. Today
the Modbus protocol is mainly used over serial communication networks and
Ethernet.
The Modbus serial communication and the Ethernet based Modbus TCP/IP
communication in this IED follow the specifications maintained by Modbus
Organization.
Modbus communication reference guides are downloadable from
Technical Resources at
www.modbus.org
.
2.1.1
Serial communication
Modbus is a master-slave protocol when it is used over serial communication
networks. This IED implements the slave side of the protocol. Depending on the
chosen physical serial interface it is possible to build multidrop networks or point-to-
point communication connections.
There can only be one Modbus master unit on a Modbus serial network. The Modbus
master unit communicates with one Modbus slave unit at a time. Usually the master
reads, or scans, data from the slaves cyclically. The master can also write data or give
commands to the slave units. Each slave unit has a unique unit address. Thus, the
master can identify the slave with which it communicates. The Modbus standard also
defines the possibility for Master broadcast transmissions.
Modbus serial protocol uses two link modes: Modbus RTU and Modbus ASCII. Both
modes are supported by this IED.
2.1.2
Ethernet communication
Modbus communication over Ethernet TCP/IP is of client-server type. This IED
operates as a Modbus server.
Modbus TCP/IP connection is established when the Modbus client opens a TCP
socket connection to the Modbus server. The socket port 502 on the TCP/IP stack is
1MRS757645 B
Section 2
Modbus overview
620 series
9
Communication Protocol Manual
