APC 6000 user manual Checking received messages on the slave side

JBUS protocol (continued)

Checking received messages on the slave side

After the master sends a request containing the slave address, the function code and data, it computes the CRC and sends it as the check word (CRC 16).

When the slave receives the request, it stores the message in memory and calculates the CRC 16 to compare it to the received CRC 16.

If the message is incorrect (unequal CRC 16 values), the slave does not respond.

If the message received is correct but the slave is unable to process it (incorrect address, incorrect data, etc.), the slave returns an error message with the following contents (see opposite):

	slave
master	slave address
	function	CRC 16
		computation
	data
	CRC 16
	CRC 16 comparison

response					Error codes:
					1.	Unknown function code
					2.	Incorrect address
					3.	Incorrect data		errors handled
					4.	Station not ready
								by the
					8.	Write error
								communication
					9.	Field overlap
								port
		function code
		received and
		MS bit = 1
slave		1						CRC 16
address
(1 à FF)
1 byte			1 byte			1 byte		2 bytes
example
01	09 H	00	00	00	00	DD CB	request
01	89 H	01	86 50				response

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