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.4
In the event that any of the errors listed in Table C.3 occur, the monitor assembles
a response message that includes the Exception Code in the Data field. The monitor
sets the Most Significant Bit in the Function Code field to indicate to the master that
the data field contains an error code rather than the requested data.
Cyclical Redundancy Check
The SEL-701-1 Monitor calculates a 2-byte CRC value using the device address,
function code, and data fields. It appends this value to the end of every Modbus
response sent. When the master device receives the response, it uses the received data
to recalculate the CRC value using the same CRC-16 algorithm. If the calculated CRC
value matches the CRC value sent by the SEL-701-1 Monitor, the master device uses
the data received. If there is not a match, the check fails and the message is ignored.
The devices use a similar process when the master sends queries.