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5.1.2
Control Byte
7
6
SAP
SDN/ DDB
locked
filter
Bits 0-
Buffer available
2
These three bits are used as a counter for resources provided externally. The FLC increments the 3 bits
as soon as a resource is available. The SPC 4-2 decrements the 3 bits when a received block was
indicated. When it receives, the SPC 4-2 reads the 3 bits after it has received the complete frame. If the
counter reading is zero, it aborts reception, sets the event flap 'No Resource' (RR, see below) and
replies with no resource (SD1 response)
Exception:
If DP mode = 1 is set, buffer available is not changed by the SPC 4-2 in DEFAULT SAP. Buffer
available must, however, be set to higher than zero otherwise the response will be no resource.
Bit 3
IN USE
The SPC 4-2 sets this bit as soon as it has entered the complete frame of a request frame in the
indication buffer. It resets it when an indication has been executed (valid or invalid). If the FLC wants to
assign a new reply block, it must wait until the bit is reset. Only then can it reload the reply update
pointer (under Mem-Lock). This prevents the FLC from reloading SPC 4-2 data when sending.
Exception:
If DP mode = 1 is set, the in-use bit is not set by the SPC 4-2 in the DEFAULT SAP. A correctly
received request frame at the DEFAULT SAP is not entered in the indication queue and not indicated.
Bit 4
RS/RA or UE
No service activated/ Service access point blocked or user error: the SPC 4-2 sets this flag when
the validation of the request SA was negative (request SA does not match received SA; in other words
the call comes from an unauthorized node). The SPC 4-2 replies with Service Access Point Blocked
[RA] in the PA mode or no service activated [RS] in the PROFIBUS mode (SD1 response). This flag
is also set when request SA = 7FH; in other words, the SAP is inactive. The SPC 4-2 replies with No
service activated [RS] (SD1 response). This bit is set as a user error [UE] is the SAP was locked. The
SPC 4-2 replies with User Error [UE] (SD1 response).
Bit 5
RR = No Resource
The SPC 4-2 sets this bit if the content of the buffer available bits is zero after receiving the frame
header; in other words, the FLC has not provided any resources or the queue is full. In both cases, the
SPC 4-2 replies with No Resource[RR] (SD1 response).
Bit 6
SDN-/DDB Filter
This bit allows the SDN-/DDB filter to be activated
0 =
The "Reply-Update-Ptr/SDN-/DDB-Tln-Tab-Ptr" pointer points to the Reply-on-Indication-Block
and therefore to the response to be sent. If the pointer = 00H, there is no response buffer and
the SPC 4-2 replies with an SRD request with a short confirmation (SC).
1 =
The "Reply-Update-Ptr/SDN-/DDB-Tln-Tab-Ptr" pointer points to the node table and the SPC 4-
2 is "Subscriber" for this SAP. The SDN-/DDB-Tln list is evaluated during a "Subscriber for
DDB-Response" access or SDN request.
Bit 7
SAP locked
The SAP is not currently accepting data. If the SPC 4-2 receives data for this SAP, it sets the event flag
user error (UE) and replies with user error (SD1 response).
Table 5-2 Control Byte
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
Bit Position
5
4
RS/RA or
RR
UE
3
2
1
IN USE
Buffer available
FLC Interface
Meaning
0
Control byte
37
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