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Modbus/TCP & Modbus RTU Function Codes
Function Code 08 - Loopback Message
Revision 4 HC900 Hybrid Controller Communications User Guide 31
9/03
4.8 Function Code 08 - Loopback Message
Description
Echoes received query message.
Query
Message can be any length up to half the length of the data buffer minus 8 bytes.
Example:
00 08 01 02 03 04
Query message format for function code 08
Slave
Address
(00 for TCP)
Function
Code
Any data, length limited to approximately half
the length of the data buffer
CRC
(RTU)
CRC
(RTU)
TCP Example 00 08 01 02 03 04
RTU Example 01 08 01 02 03 04 CRC CRC
Response
Example:
00 08 01 02 03 04
Response message format for function code 08
Slave
Address
(00 for TCP)
Function
Code
Data bytes received CRC
(RTU)
CRC
(RTU)
TCP Example 00 08 01 02 03 04
RTU Example 01 08 01 02 03 04 CRC CRC