701 Motor Protection Relay Date Code 20010719
Modbus
®
RTU Communications Protocol
Modbus RTU Communications Protocol
C.2
Modbus RTU Communications Protocol
Modbus Queries
Modbus RTU master devices initiate all exchanges by sending a query. The query
consists of the fields shown in Tab le C.1.
Table C.1 Modbu s Query Fields
The SEL-701 Relay SLAVEID setting defines the device address when the relay
rear-panel port is set for Modbus communication. 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-701 Relay are described in Ta bl e C. 2. The
cyclical redundancy check is an error detection method that validates the data received
by the slave device and helps ensure that the packet received is identical to the packet
sent by the master device. The CRC-16 Cyclical Redundancy Check algorithm is used.
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.
Table C.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 2 bytes
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