Siemens SIMATIC NET SPC 4-2 LF, Manual, Page: 47 / 178

FLC Interface
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
47
5.4 Reply-on-Indication Blocks
5.4.1 Description
Function
The FLC must provide the reply data in the buffers of the reply-on-indication
blocks. If reply data are requested, the SPC 4-2 fetches the reply update pointer
from the relevant SAP lists and sends the data from the reply buffer. Once the job
is completed, the SPC 4-2 indicates the job by entering the status (valid indication)
in the response header, setting the write pointer to the next free segment, and
generating the IND interrupt.
A job is completed and indicated when:
● An SM frame, an SDN or a DDB response frame has been received free of
error and validated.
● An SDA or SRD frame has been received error-free and validated, the
response has been sent and the next request frame to another node or (with
toggled FCB/FCV bits) to the local station address has been correctly received.
If the SPC 4-2 receives an SRD or DDB request frame with a net data length = 0
and if the response data length is also = 0, the SPC 4-2 does
not enter this frame
in the indication queue and does not indicate it (empty polling).
The FLC can control how often the data are sent from the indication reply buffer by
setting a bit in the responder status (byte 2). If bit (4): single update reply is set in
the "resp-status" of the indication reply buffer, a reply in the indication reply buffer
is sent only once. If this bit = logical "0", the SPC 4-2 sends the buffer again
(multiple update reply) with each call frame to this SAP.
The least significant nibble (lower 4 bits) specifies whether the job is sent with high
or low priority.