Siemens CPU 948 User Manual

Cycle time monitoring The CPU monitors the cycle time in case it exceeds a maximum value.
The standard setting for this maximum value is 200 ms. You can set the
cycle time monitoring yourself or restart it during user program execution
(refer to DX 0/Chapter 7 and special function OB OB 222/Section 6.16).
Process input and output
image (PII and PIQ)
The process image of the inputs and outputs is a memory area in the
internal RAM.
Before cyclic execution of the user program begins, the system
program reads the signal states of the input peripheral modules and
transfers them to the process input image. The user program evaluates
the signal states in the process input image and then sets the
appropriate signal states for the outputs in the process output image.
After the user program has been processed, the system program
transfers the signal states of the process output image to the output
peripheral modules.
Buffering the I/O signals in the process image of the inputs and
outputs avoids a change in a bit within a program cycle from causing
the corresponding output to "flutter".
The process image is therefore a memory area whose contents are
output to the peripherals and read in from the peripherals once per
cycle.
Note
The process image only exists for input and output bytes of the "P"
peripherals with byte addresses from 0 to 127!
Apart from the process image integrated in the system, you can use
OB 126 to define and transfer further process images (refer to
Section 6.6)
Interprocessor communication
(IPC) flags
IPC flags exchange data between individual CPUs (multiprocessing) or
between the CPU and some communication processors.
The system program reads the input IPC flags of the CPU before cyclic
execution of the user program begins. After the STEP 5 program is
processed, the system program transfers the output IPC flags to the
coordinator or to the communications processors.
You define the input and output IPC flags when you create data block
DB 1 (refer to Section 10.1.6).
Processing the User Program
CPU 948 Programming Guide
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Siemens CPU 948 Specifications

General

Brand	Siemens
Model	CPU 948
Category	Controller
Language	English