Date Code 20110408 Instruction Manual SEL-551 Relay
Appendix G
Modbus RTU Communications Protocol
Overview
This appendix describes Modbus
®
RTU communications features supported
by the SEL-551 Relay. Complete specifications for the Modbus protocol are
available from the Modicon website 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-551 Modbus communication allows a Modbus master device to:
➤ Acquire metering, monitoring, and event data from the relay.
➤ Control SEL-551 output contacts.
➤ Read the SEL-551 self-test status and learn the present
condition of all relay protection elements.
Modbus RTU Communications Protocol
Modbus Queries
Modbus RTU master devices initiate all exchanges by sending a query. The
query consists of the fields shown in Table G.1.
The SEL-551 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.
Function codes supported by the SEL-551 are described in Table G.2.
Table G.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
To Next Page
Loading...
