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5 MODBUS FUNCTIONS…
5.5 Loopback Test – Function Code 08
5.5.1 Loopback Test Query
The Loopback Test Query tests the MODBUS system and does not affect the
operation of the slave. Variations in the response may indicate faults in the
MODBUS system. The information field contains two bytes for the designation of
the diagnostic code followed by two bytes to designate the action to be taken.
Example
Data Data Error
Address Function Diagnostic Diagnostic Data* Data* Check (CRC-16)
Code High Code Low Field
01 08 00 00 A5 37 DA 8D
*These are considered to be the information fields for the diagnostic mode.
5.5.2 Loopback Test Response
The Loopback Test Response always echoes the query, only diagnostic code 0
(bytes 3 and 4) can be used.
Example
Data Data Error
Address Function Diagnostic Diagnostic Data Data Check (CRC-16)
Code High Code Low Field
01 08 00 00 A5 37 DA 8D
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