Rockwell Automation Publication 825-UM004D-EN-P - November 2012
135
Modbus RTU Communications
Chapter
10
Modbus Queries
Modbus RTU master devices initiate all exchanges by sending a query.
The query consists of the fields shown in Table 65.
Table 65 - Modbus Query Fields
The 825-P 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.
The cyclical redundancy check detects errors in the received data. If an error is
detected, the relay discards the packet.
Modbus Responses
The slave device sends a response message after it performs the action
requested in the query. If the slave cannot execute the command for any
reason, it sends an error response. Otherwise, the slave device response
is formatted similarly to the query including the slave address, function
code, data (if applicable), and a cyclical redundancy check value.
Supported Modbus
Function Codes
The 825-P supports the Modbus function codes shown in Table 66.
Table 66 - 825-P Modbus Function Codes
Field
Number of Bytes
Slave Device Address
1 byte
Function Code
1 byte
Data Region
0…250 bytes
Cyclical Redundancy Check (CRC)
2 bytes
Codes
Description
03h
Read Holding Registers
06h
Preset Single Register
10h
Preset Multiple Registers
60h
Read Parameter Information
61h
Read Parameter Text
62h
Read Enumeration Text
7Dh
Encapsulate Modbus Packet With Control
77Eh
NOP
Summary of Contents for 825-P
Page 1: ...Modular Protection System for Motors Catalog Number 825 P User Manual...
Page 4: ...4 Rockwell Automation Publication 825 UM004D EN P November...
Page 64: ...64 Rockwell Automation Publication825 UM004D EN P November 2012 Chapter 5 Using MPS Explorer...
Page 234: ...234 Rockwell Automation Publication 825 UM004D EN P April 2012 Chapter B ParameterList...
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