13
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