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C
C-19GFK–0804B Appendix C RTU Protocol
5. The register number field specifies a register not available in theMicro PLC (returned
for function code 6).
6. The analog input number field specifies an analog input number not available in the
Micro PLC (returned for function code 3).
Invalid Data Value Error Response (3)
An error response with a subcode of 3 is called an invalid data value error response. This
response is sent in the following cases:
The first byte of the data field is not equal to 0 or 255 (FFh) or the second byte of the data
field is not equal to 0 for the Force Single Output Request (Function Code 5).
This response is also sent when the data length specified by the memory address field is
longer than the data received.
Query Processing Failure Error Response (4)
An error response with a subcode of 4 is called a query processing failure response. This
error response is sent by the Micro PLC if it properly receives a query but communica-
tion between it and the computer fail.
Serial Link Timeout
The only cause for a RTU device to time out is if an interruption to a data stream of 3
character times occurs while a message is being received. If this occurs the message is
considered to have terminated and no response will be sent to the master. There are
certain timing considerations due to the characteristics of the slave that should be taken
into account by the master.
After sending a query message, the master should wait approximately 500 milliseconds
before assuming that the slave did not respond to its request.
Invalid Transactions
If an error occurs during transmission that does not fall into the category of an invalid
query message or a serial link time–out, it is known as an invalid transaction. Types of
errors causing an invalid transaction include:
Bad CRC.
The data length specified by the memory address field is longer than the data
received.
Framing or overrun errors.
Parity errors.
If an error in this category occurs when a message is received by the Micro PLC, theMir-
co PLC does not return an error message. The Micro PLC treats the incoming message
as though it was not intended for it.