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2-12 Series 90-30/20/Micro Programmable Controllers Reference Manual
–
September 1998 GFK-0467K
2
PCM Communications with the PLC
(Models 331 and Higher)
There is no way for intelligent option modules (IOM), such as the PCM, to interrupt the CPU
when they need service. The CPU must poll each intelligent option module for service requests.
This polling occurs asynchronously in the background during the sweep (see flow chart below).
When an intelligent option module is polled and sends the CPU a service request, the request is
queued for processing during the system communications window.
YES
POLL NEXT IOM
QUEUE REQUEST
ALL
RECEIVED
NO
NO
YES
a43067
START
STOP POLLING
?
POLLED
IOMS
?
REQUEST
Figure 2-4. PCM Communications with the PLC