Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 119)

Calculating Program Timing
Chapter 9
9-2
processor internal checks
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
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
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
Effects of False versus True Logic on Scan Time
The rung below—which changes states from one program scan to the
next—changes your program scan time by about .25 ms.
MASKED MOVE
Source
Mask
Dest
N7:0
1100110011001100
N7:2
I:000
00
MVM
If I:000/00 is: Then the rung is:
On
True and the processor executes the maskedĆmove instruction. A maskedĆmove
instruction takes 258 µ s to execute (see appendix A of the PLCĆ5 Programming
Software Instruction Set Reference, publication 6200Ć6.4.11).
Off False and the processor scans the rung but does not execute it. It takes only 1.4
microseconds to just scan the rung.
Other instructions may have a greater or lesser effect.
Effects of Different Instructions on Logic Scan Time
Some instructions have a much greater effect on logic scan time than
others based on the time it takes to execute that instruction.
Program scan time is also affected by the basic construction of your ladder
rungs. The sizes of rungs and the number of branches in each can cause
the scan time to fluctuate greatly.