RadiSys 6200plus Series Bedienungsanleitung PDF herunterladen (Seite 113)

Performance and Theory of Operations

Chapter 7

7-8

Program Scanning

The program scan cycle is the time it takes the processor to execute the

logic scan once, perform housekeeping tasks, and then start executing

logic again.

The processor continually performs logic scanning and housekeeping. In a

PLC-5/V40, for example, basic housekeeping takes 3.2 ms. If it takes the

processor 21.8 ms to execute a logic scan, the overall program scan cycle

is 25 ms. You can monitor the program scan time using the processor

status screen.

Housekeeping activities for PLC-5/VME processors include:

processor internal checks

updating the input image table with remote I/O input status as contained

in the remote I/O buffer

updating the remote I/O buffer with output data from the output

image table

Housekeeping activities for other standard PLC-5 processors also include:

updating the input image table with processor-resident I/O input status

updating processor-resident local I/O output modules with data from the

output image table

Important: PLC-5/V40L processors also scan extended-local I/O chassis

(on channel 2) during housekeeping.

If no change in input status occurs and the processor continues to execute

the same logic instructions, the program scan cycle remains consistent (in

our example, at 25 ms). In real systems, however, the program scan cycle

fluctuates due to the following factors:

false logic executes faster than true logic

different instructions execute at different rates

different input states cause different sections of logic to be executed

interrupt programs affect program scan times

Housekeeping

Logic Scan

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RadiSys 6200plus Series Bedienungsanleitung Seite 113