Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 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