Date Code 20080918 Instruction Manual SEL-749M Relay
Appendix D
Modbus RTU Communications Protocol
Instructi on Manual
Overview
This appendix describes Modbus
®
RTU communications features supported
by the SEL-749M Relay on communications Port 3 or Port 4.
Complete specifications for the Modbus protocol are available from the
Modbus user’s group website at www.modbus.org.
Enable Modbus protocol with the serial port settings. When Modbus protocol
is enabled, the relay switches the port to Modbus protocol and deactivates the
ASCII protocol.
Modbus RTU is a binary protocol that permits communication between a
single master device and multiple slave devices. The communication is half
duplex; only one device transmits at a time. The master transmits a binary
command that includes the address of the desired slave device. All of the slave
devices receive the message, but only the slave device with the matching
address responds.
The SEL-749M Modbus communication allows a Modbus master device to do
the following:
➤ Acquire metering, monitoring, and event data from the relay.
➤ Control SEL-749M output contacts.
➤ Read the SEL-749M self-test status and learn the present
condition of all the relay protection elements.
➤ Read most of the relay settings and modify the relay settings.
Communications Protocol
Modbus Queries
Modbus RTU master devices initiate all exchanges by sending a query. The
query consists of the fields shown in Table D.1.
Table D.1 Modbus Query Fields
Field Number of Bytes
Slave Device Address 1 byte
Function Code 1 byte
Data Region 0–251 bytes
Cyclical Redundancy Check (CRC) 2 bytes
Courtesy of NationalSwitchgear.com
To Next Page
Loading...
