Date Code 20090730 Instruction Manual SEL-734 Meter
Appendix F
Modbus RTU Communications Protocol
Overview
This appendix describes Modbus
®
RTU communications features supported
by the SEL-734 Meter communications port.
Complete specifications for the Modbus protocol are available from the
Modicon web site at www.modicon.com.
Modbus RTU is a binary protocol that permits communication among 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-734 Modbus communication allows a Modbus master device to do
the following:
➤ Acquire metering, monitoring, load profile, and event data from
the meter.
➤ Control SEL-734 output contacts and remote bits.
➤ Read the SEL-734 self-test status and learn the present
condition of all the meter 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 F.1.
The SEL-734 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.
A cyclical redundancy check detects errors in the received data. If an error is
detected, the meter discards the packet.
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
Courtesy of NationalSwitchgear.com
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