Typical Mode Of Operation Of A Cpu - Siemens CPU 948 Programming Manual

1.2

Typical Mode of Operation of a CPU

Mode of operation of a CPU
1.
Cyclic processing
Cyclic processing
Phase Sequence
All the input modules assigned to the
1 CPU are scanned by the system
program and the values read in are
stored in the process image of the
inputs (PII).
The values contained in the PII are
2 processed by the user program and the
values to be output are entered in the
process image of the outputs (PIQ).
The values contained in the process
3 image of the outputs are output by the
system program to the output modules
assigned to the CPU.
CPU 948 Programming Guide
C79000-G8576-C848-04
The following modes of operation are possible in a CPU:
2.
Time-controlled processing
This is the main part of all activities in the CPU. As the name already
says, the same operations are repeated in an endless cycle.
Cyclic processing can be divided into three main phases, as follows:
Typical Mode of Operation of a CPU
3.
Interrupt-driven processing
CPU
Read in process image
of the inputs
Evaluate input signals,
set output signals
I 1.5 &
I 1.6
= 1
I 1.4 &
I 1.3
Q 3.1
Output process image
of the outputs
Process
Input I 1.3
Input I 1.4
Input I 1.5
Output Q 2.0
Output Q 3.1
Output Q 4.7
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