E.2
SEL-751A Relay Instruction Manual Date Code 20100129
Modbus RTU Communications
Communications Protocol
Communications Protocol
Modbus Queries
Modbus RTU master devices initiate all exchanges by sending a query. The
query consists of the fields shown in Table E.1.
The SEL-751A 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 the
query specifies. If the slave cannot execute the query command for any reason,
it sends an error response. Otherwise, the slave device response is formatted
similarly to the query and includes the slave address, function code, data (if
applicable), and a cyclical redundancy check value.
Supported Modbus
Function Codes
The SEL-751A supports the Modbus function codes shown in Ta bl e E.2 .
Table E.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
Table E.2 SEL-751A Modbus Function Codes
Codes Description
01h Read Discrete Output Coil Status
02h Read Discrete Input Status
03h Read Holding Registers
04h Read Input Registers
05h Force Single Coil
06h Preset Single Register
08h Diagnostic Command
10h Preset Multiple Registers
60h Read Parameter Information
61h Read Parameter Text
62h Read Enumeration Text
7Dh Encapsulate Modbus Packet With Control
7Eh NOP (can only be used with the 7Dh function)
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