RKC INSTRUMENT PG500, Communication Instruction Manual

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4. MODBUS COMMUNICATION PROTOCOL
IMR02F04-E1
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4.4 Slave Responses
(1) Normal response

In the response message of the Read Holding Registers, the slave returns the read out data and the
number of data items with the same slave address and function code as the query message.

In the response message of the Preset Single Register, the slave returns the same message as the
query message.

In the response message of the Diagnostics (Loopback test), the slave returns the same message as
the query message.
(2) Defective message response

If the query message from the master is defective, except for transmission error, the slave returns the
error response message without any action.
Slave address
Function code
Error code
Error check CRC-16
Error response message

If the self-diagnostic function of the slave detects an error, the slave will return an error response
message to all query messages.

The function code of each error response message is obtained by adding 80H to the function code of
the query message.
Error code Contents
1 Function code error (An unsupported function code was specified)
2 When the mismatched address is specified.
3
When the specified number of data items in the query message exceeds the
maximum number of data items available
4 Self-diagnostic error response
(3) No response
The slave ignores the query message and does not respond when:

The slave address in the query message does not coincide with any slave address settings.

The CRC code of the master does not coincide with that of the slave.

Transmission error such as overrun, framing, parity and etc., is found in the query message.

Data time interval in the query message from the master exceeds 24-bit time.