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Wrong PDU
A.3.4 Commad code 0x08Diagnostic function
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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.
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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.
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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.
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Request PDU
Respond PDU
Wrong PDU
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
request (such as the register address does not exist, etc.), the slave station will return
an abnormal response to inform the master of the actual situation.
Abnormal response command code = normal response command code + 0x80,
Abnormal code value and meaning as shown in the following table
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