Siemens CPU 948 - Why Data Is Buffered

10.2.3
Why Data is Buffered Generally, the multiprocessor mode is used to distribute tasks on
several CPUs. Since the tasks are not identical and the performance of
the CPUs involved can be different, the program execution of the
individual CPs in the multiprocessor mode is always asynchronous.
This means that the data sent by a CPU cannot always be received
immediately by another CPU.
For this reason, the data to be transferred is buffered on the
coordinator 923 C. The number of the sender and receiver are always
included along with the data.
Example
Data transfer from CPU 3 to CPU 2:
1st step:
CPU 3 buffers its data on the coordinator.
2nd step:
When CPU 2 is ready to receive, it copies the data from the coordinator
buffer to the destination DB.
C
O
R
C
C
P
U
1
C
P
U
2
C
P
U
3
C
P
C
P
I
M
..
..
I
SEND, parameter of receiving CPU = 2
QII Q
C
O
R
C
C
P
U
1
C
P
U
2
C
P
U
3
C
P
C
P
I
M
..
..
I
RECEIVE, parameter of transmitting CPU = 3
QII Q
Multiprocessor Communication
CPU 948 Programming Guide
C79000-G8576-C848-04
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