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Access-Value = 08H filters all request frames except for
●
Send and Request Data low (SRD-low),
●
Send and Request Data high (SRD-high),
●
Send and Request Data with DDB (DDB-Request),
●
DDB-Response low,
●
DDB-Response high.
Since the filter allows DDB-Response-low/high frames to pass, this means that the
SPC 4-2 as a DP slave can also listen on the bus using the DEFAULT SAP and
evaluate the received data. Since the publisher is generally not the DP master, the
SDN/DDB filter should be activated. If the SDN/DDB filter bit in the control byte of
the DEFAULT SAP is set, the source address (SA) and SSAP are validated solely
in the SDN/DDB node list. Received DDB response frames are entered in the
indication queue. Following this, the "IND-WP-PRE" and "IND-WP" pointers are set
to the next free segment and the "IND-PRE" and "IND" interrupts are generated.
If an SRD-low/high frame is received from the DP master and there are no input
data present (in other words response buffer length = 0), the SPC 4-2 responds as
follows:
●
either with SC if 08H is entered in the response status of the DEFAULT SAP
(in other words low priority).
●
or with an SD2 frame with a length LE = 4 (in other words net data length = 1)
if 0AH is entered in the response status of the DEFAULT SAP (in other words
high priority). SPC4-1 and SPC4-2 send 00H as a dummy byte, with SPC4
this was FFH.
If a DDB request is received from the DP master and there are no input data
available (in other words, response buffer length = 0), the SPC 4-2 responds with
"No-Service-Activated (RS)" and sets the RS flag in the control byte of the
DEFAULT SAP.
56
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
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