Schweitzer Engineering Laboratories SEL-701-1, Instruction Manual

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SEL-701-1 Monitor
Date Code 20011009
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 Table C.1 .
The SEL-701-1 Monitor SLAVEID setting defines the device address when the
monitor 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-701-1 Monitor are described in Table 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