GE 90-30/20/Micro manual PCM Communications with the PLC Models 331 and Higher

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.

START

ALL

IOMS

POLLED

?

NO

POLL NEXT IOM

NO REQUEST RECEIVED

?

a43067

YES

STOP POLLING

YES

QUEUE REQUEST

Figure 2-4. PCM Communications with the PLC

2-12 Series 90-30/20/Micro Programmable Controllers Reference Manual– September 1998 GFK-0467K