Modbus RTU Communications Protocol
F-1
SEL-300G Instruction Manual
Date Code 20060731
APPENDIX F: MODBUS RTU COMMUNICATIONS PROTOCOL
I
NTRODUCTION
This appendix describes Modbus
®
RTU communications features supported by the SEL-300G
Relay communications port.
Complete specifications for the Modbus protocol are available from the Modicon web site at
www.modicon.com.
Enable Modbus protocol using 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-300G Modbus communication allows a Modbus master device to:
•
Acquire metering, monitoring, and event data from the relay.
•
Control SEL-300G output contacts and remote bits.
•
Read the SEL-300G self-test status and learn the present condition of all relay protection
elements.
M
ODBUS
RTU C
OMMUNICATIONS
P
ROTOCOL
Modbus Queries
Modbus RTU master devices initiate all exchanges by sending a query. The query consists of the
fields shown in Table F.1.
Table F.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
The SEL-300G SLAVEID setting defines the device address. Set this value to a unique number
for each device on the Modbus network. For Modbus communication to operate properly, no
two slave devices may have the same address.
Summary of Contents for SEL-300G
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