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5.3
Indication Queue
5.3.1
Description
Function
If the SPC 4-2 receives a frame, it enters the frame header in the indication queue
and then checks the free length in the queue (this is possible because one
segment must always remain free). If at least one segment (8 or 16 bytes) is free
(in addition to the special free segment), it continues reception and enters the data
in the queue as long as there is free memory available. When a request frame
(call) is received, the SPC 4-2 validates the frame header characters with the
values it has set in the SAP list. The structure is described Table 5-1 SAP List.
The indication queue is managed as a ring buffer with read (IND-RD) and write
(IND-WR) pointers. The SPC 4-2 is responsible for the write pointer and the FLC is
responsible for the read pointer.
The IND-WR-PRE pointer allows fast slave reactions (e.g. for DP). With a suitable
parameter assignment, an indication interrupt is generated after correct reception
of the request frame and not at the end of the next frame to another node.
44
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
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