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
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