Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 467)

A–B
D
Appendix
D–1
Estimating Scan Time
This appendix:
•
contains worksheets that allow you to estimate the scan time for your
particular controller configuration and program
•
includes scan time calculation for an example controller and program
Use the instruction execution times listed in appendix C.
The diagram and table below breaks down the processor operating cycle into
events. Directions for calculating the scan time of these events appear in the
worksheets.
Processor Overhead
Communication
Output Scan
Program Scan
Input Scan
Events in the processor operating cycle
Event Description
Input Scan
The status of input modules is read and the input image in
the processor is updated with this information.
Program Scan
The ladder program is executed. The input image table is
evaluated, ladder rungs are solved, and the output image is
updated. The information is not yet transferred to the
output modules.
Output Scan
The output image information is transferred to the output
modules.
Communications
Communication with programmers and other network
devices takes place.
Processor Overhead
Processor internal housekeeping takes place. Actions
include performing program pre–scan and updating the
internal timebase and the Status file.
Events in the Operating Cycle