Date Code 20020903
SEL-587Z Instruction Manual
Appendix D
Modbus RTU Communications Protocol
Modbus Overview
This appendix describes Modbus
®
RTU communications features supported
by the SEL-587Z Relay. 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-587Z Modbus communication allows a Modbus master device to:
Acquire metering, monitoring, and event data from the relay.
Control SEL-587Z output contacts.
Read the SEL-587Z self-test status and learn the present
condition of all relay protection elements.
Queries
Modbus RTU master devices initiate all exchanges by sending a query. The
query consists of the fields shown in
The SEL-587Z 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-587Z are described in
The CRC detects errors in the received data. If an error is detected, the relay
discards the packet.
Table D.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
Содержание SEL-587Z
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