Date Code 20100129
Instruction Manual
SEL-751A Relay
Appendix E
Modbus RTU Communications
Overview
This appendix describes Modbus
®
RTU communications features supported
by the SEL-751A Feeder Protection Relay. Complete specifications for the
Modbus protocol are available from the Modbus user’s group website at
www.modbus.org.
Enable Modbus TCP protocol with the optional Ethernet port settings. The
SEL-751A supports up to two Modbus TCP sessions. The TCP port number
for each session is selected with the Ethernet port settings. The default TCP
port number is the Modbus TCP registered port 502. Modbus TCP uses the
device IP address as the Modbus identifier and accesses the data in the relay
using the same function codes and data maps as Modbus RTU.
Enable Modbus RTU protocol with the serial port settings. When Modbus
RTU protocol is enabled, the relay switches the port to Modbus RTU 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-751A Modbus communication allows a Modbus master device to do
the following:
➤
Acquire metering, monitoring, and event data from the relay.
➤
Control SEL-751A output contacts.
➤
Read the SEL-751A 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.
Summary of Contents for 751A
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