Date Code 20170927 Instruction Manual SEL-751 Relay
Appendix E
Modbus RTU Communications
Overview
This appendix describes Modbus RTU communications features supported by
the SEL-751 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-751 supports as many as 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 slave device you want. All of the
slave devices receive the message, but only the slave device with the matching
address responds.
The SEL-751 Modbus communication allows a Modbus master device to do
the following:
➤ Acquire metering, monitoring, and event data from the relay.
➤ Control SEL-751 output contacts.
➤ Read the SEL-751 self-test status and learn the present
condition of all the relay protection elements.
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