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Date Code 20081022
Instruction Manual
SEL-787 Relay
Appendix E
Modbus RTU Communications
Instructi on Manual
Overview
This appendix describes Modbus
®
RTU communications features supported
by the SEL-787 Transformer Protection Relay. Complete specifications for the
Modbus protocol are available from the Modbus user’s group website at
www.modbus.org.
Modbus TCP is automatically available with the optional Ethernet port.
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. The TCP Port for Modbus TCP is 502.
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-787 Modbus communication allows a Modbus master device to do
the following:
➤
Acquire metering, monitoring, and event data from the relay.
➤
Control SEL-787 output contacts.
➤
Read the SEL-787 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.
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