Burkert Type 8717 - Manual (Page 68)

68
Starting up the Modbus
exceptional reply
If a master device sends a request to a slave device, the master device expects a normal reply When the master
has transferred a request, one of the four events may occur:
• If the slave device receives the request without a data transfer error and the request can be processed normally,
a normal reply is sent back.
• If the slave device does not receive the request due to a data transfer error, no reply is sent back. The master
device program determines a timeout for the request.
• If the slave device determines a data transfer error, no reply is sent back. The master device program deter-
mines a timeout for the request.
• If the slave device receives the request without a data transfer error, but the request cannot be processed (e.g.
to read out a non-existent register), an exceptional reply is sent back which informs the master device about the
type of error. The exceptional reply has two fields that distinguishes it from a normal reply.
function code field
If the answer is normal, the slave sends back a copy of the function code included in the original request in the
appropriate field of the reply. If the reply is an exception, the value of the function code is exactly 0x80 hexa-
decimal numbers higher than it would be in a normal reply.
Data field
If the reply is an exception, the slave sends an exception code in the data field, that defines the operating status
of the slave which caused the exception.
example of an exceptional reply
Request (Read Input Register 0x68) register is outside the validity range
field name value
Slave address 0x01
Function 0x04
Initial address high 0x00
Initial address low 0x68 (invalid register)
Number of high registers 0x00
Number of low registers 0x01
Error check CRC (high byte)
Error check CRC (low byte)
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MFC Family