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1MAC052634-MB D
Section 2
2.1
2.1.1
2.1.2
615/620 series ANSI
Communication Protocol Manual
Modbus overview
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.
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.
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 reserved for
Modbus. If the connection request is accepted by the server, the client can start
communicating with the server unit.
IEDs can usually accept several simultaneous Modbus TCP/IP client connections even
though the number of connections is limited. It is possible to configure the IED to only
accept socket connection requests from known client IP addresses.
Section 2
Modbus overview
7
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