У вас неполное описание Modbus
Цитата:
6.8 08 (0x08) Diagnostics (Serial Line only)
MODBUS function code 08 provides a series of tests for checking the communication system between a client ( Master) device and a server ( Slave), or for checking various internal erroconditions within a server.
The function uses a two–byte sub-function code field in the query to define the type of test to be performed. The server echoes both the function code and sub-function code in a normal response. Some of the diagnostics cause data to be returned from the remote device in the data field of a normal response.
...
Sub-function 00 - Return Query Data
|