Rockwell Automation Publication 825-UM004D-EN-P - November 2012 131
Chapter 10
Modbus RTU Communications
Overview
This chapter describes the communications features supported by the 825-P
Modbus® RTU option card.
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 825-P Modbus communication allows a Modbus master device to do the
following:
• A
cquire metering, monitoring, and event data from the relay.
• Control 825-P output contacts and remote bits.
• Read the 825-P self-test status and learn the present condition of
al
l the relay protection elements.
Installation
Mounting
The optional 825-PMB Modbus Communication Card occupies the
communication expansion slot [C] in the 825-P Modular Protection
System.
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