Siemens SIMATIC NET SPC 4-2 LF Manual page 83

If "Spec-Clear-Mode = 1" is set, the SPC 4-2 also accepts data frames without
output data in the "Data Exchange" state of the DP controller. This applies
regardless of the value of the indication buffer length set in the default SAP of the
SPC 4-2. If "Spec-Clear-Mode = 1" is set, the SPC 4-2 performs the following
actions after receiving a frame with a net data length = 0 (even if the selected
indication buffer length > 0):
- Generates "Watch-Dog-Reset" interrupt.
- Generates the "Output-Data-Exchange" interrupt.
- Replies with input data.
- Sets IND-N-Valid := 1 and IND-N-Cleared := 1
If the user updates the output data by swapping indication buffers N and U, "IND-
N-Valid = 1" and "IND-N-Cleared = 1" indicate that the output data must be
replaced by the corresponding clear coding. The data received as a subscriber
from DDB response frames that are located in the indication queue must also be
replaced by the corresponding clear coding. In the clear state, the output data are
not deleted by the SPC 4-2; in other words not overwritten by 00H.
If the SPC 4-2 receives a "Read-Output-Data" frame, it checks the "IND-U-Cleared"
bit. If "IND-U-Cleared=1" is set, the SPC 4-2 replies with 00H as the output data.
With the "Spec-Clear-Mode = 1" setting, the SPC 4-2 does not delete the output
data even when it executes Leave-Master or when it receives a global control
frame with "Clear-Data = 1". To indicate the clear state, the SPC 4-2 sets "IND-N-
Valid = 1" and "IND-N-Cleared = 1".
If "Spec_Clear_Mode = 0" is set, the SPC 4-2 behaves like the SPC 4. Fail-safe
frames are not detected; in the clear state, the output data are overwritten with
00H. IND-N-Cleared is not set. IND-U-Cleared is don't care.
Notice:
The execution time t
on the indication buffer length "n" and the baud rate:
At 12 Mbauds this is approximately (15 + n) bit clocks < t
clocks.
For baud rates lower than 3 Mbauds it is approximately t
clocks.
This means that the SPC 4-2 requires approximately 150 bit clocks to clear 256
bytes at 31.25 kbauds. Since the bus parameter T
significantly shorter at this baud rate, request frames to the SPC 4-2 can be lost.
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
required by the SPC 4-2 to clear the output data depends
CLR
ASIC Interface
< (20 + 1.5n) bit
CLR
= ( 20 + 0.5n) bit
CLR
(= 37 bit clocks) is
ID1
83