701 Motor Protection Relay Date Code 20010719
Modbus
®
RTU Communications Protocol
Modbus RTU Communications Protocol
C.4
In the event that any of the errors listed in Tab le C.3 occur, the relay assembles a
response message that includes the Exception Code in the Data field. The relay 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 Relay 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 Relay, 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.
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