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
Address
Function
01
08
*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
Address
Function
01
08
Data
Data
Diagnostic
Diagnostic
Code High
Code Low
00
00
Data
Data
Diagnostic
Diagnostic
Code High
Code Low
00
00
5 MODBUS FUNCTIONS...
Error
Data*
Data*
Check
Field
A5
37
DA
Error
Data
Data
Check
Field
A5
37
DA
(CRC-16)
8D
(CRC-16)
8D
13