Starter kit for ecomatController EC2121
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4
Features of a PLC
Content
Cyclical program processing ..................................................................................................................22
Time response ........................................................................................................................................23
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This chapter describes some basics of programmable logic controllers. Should you have some
knowledge of the operation of a PLC, you can skip this chapter.
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4.1
Cyclical program processing
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Characteristic of PLC systems is the cyclical program processing, i.e. after processing of the
application program, processing is automatically started from the beginning.
This ensures that the input signals are read continuously. Changes are found and considered in the
application program.
If input signals of the switches and sensors change during processing of the application program, this
may lead to incorrect results for the output signals. Therefore the input signals are copied to a
clipboard (process image of the inputs - PII) at the beginning of the cycle.
The application program then only works with the signals from the PII when the inputs are read. The
results for the outputs are also written to the clipboard (process image of the outputs - PIO) during
processing.
When processing of the application program is completed (end of cycle), the signals are transferred
from the PIO to the outputs and the actuators switch accordingly.
