Veikong VFD500 Series Operation Manual page 107

Appendix A Modbuscommunication protocol

Respond PDU
Command code
Initial address
Number of register

Wrong PDU
Command code
Abnomal Code
A.3.4 Commad code 0x08Diagnostic function

Modbus Command Code 0x08 Providea series of tests to check the communication system
between the client (master) device and the server (slave) or various internal error conditions in the
server.

This function uses the sub-command code of 2 bytes inquery to define the type of test to be
performed. The server copies the command and subcommand codes in the normal response.
Some diagnostics cause the remote device to return the data through the normally responding data
fields.

Diagnostic functions to remote devices generally do not affect the user program running in the
device. The main diagnostic function of this product is not line diagnosis (0000), used to test the
host from the machine is normal communication.

Request PDU
Command code
Subcommand code
Data
Respond PDU
Command code
Subcommand code
Data
Wrong PDU
Command code
Abnomal code
A.4 Abnormal response information
When the master device sends a request to the slave device, the master expects a normal
response. The master's query may result in one of four events:
(1) If the slave device receives a request for a communication error and the query can be
processed normally, the slave device will return a normal response.
(2) If the slave device does not receive the request due to a communication error, no
information can be returned and the slave device times out.
(3) If the slave device receives a request and detects a communication error (parity,
address, framing error, etc.), no response is returned and the slave device times out.
(4) If the slave device receives no communication error request, but can not handle the
VFD500 high performance vector control frequency inverter user manual
in front,first send initial
address'register value
1byte 0x10
0x0000 ～ 0xFFFF( high
2byte
8 bit in front)
2byte 1～16(1～16, high 8 bit
in front)
1byte 0x90
1byte See Abnormal response
information
1byte 0x08
2byte 0x0000～0xFFFF
2byte 0x0000～0xFFFF
1byte 0x08
2byte 0x0000
2byte Same as request of PDU
1byte 0x88
1byte See Abnormal response
information
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