Date Code 20170814
Instruction Manual
SEL-700G Relay
Appendix E
Modbus RTU Communications
Instruction Manual
Overview
Enable Modbus TCP protocol with the optional Ethernet port settings. The
SEL-700G 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 by 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 necessary slave device. All of the
slave devices receive the message, but only the slave device with the matching
address responds.
The SEL-700G Modbus communication allows a Modbus master device to do
the following:
➤
Acquire metering, monitoring, and event data from the relay.
➤
Control SEL-700G output contacts.
➤
Read the SEL-700G self-test status and learn the present
condition of all the relay protection elements.
Communications Protocol
Modbus Queries
Modbus RTU master devices initiate all exchanges by sending a query. The
query consists of the fields shown in Table E.1.
Table E.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
Summary of Contents for SEL-700G Series
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