Allen-Bradley ClutchBrake PLC-5 Series - Programming Considerations for PLC 5 Processors; Purpose of this Appendix; Program Scan: How the Processor Scans a Program

Appendix F
Publication 65566.5.1 - October 1996
Programming Considerations
for PLC5 Processors
The purpose of this appendix is to help you make correct decisions
when writing ladder logic for controlling machine functions.
We divided this guide into these sections:
• Program Scan: How the Processor Scans a Program
• What Affect Does Your Ladder Logic Have on Program Scan?
• How to Program a Faster Response
• What are Subroutines and Why Use Them?
• Working with Data
• Troubleshooting the Clutch/Brake Controller
For additional information on these topics, refer to the 1785 PLC-5
Programmable Controllers User Manual, publication 1785-6.5.12.
What is the Program Scan?
The program scan is the mechanism by which the processor operates.
It is sequential, occurring as fast as 100 times per second.
Here is what the processor does during the program scan (Figure F.1):
Figure F.1
The
program scan lets the processor monitor inputs and control
outputs based on decisions programmed by your ladder logic.
Input Image
Output Image
Input Module Output Module
Signals from input devices such as
proximity switches
photoelectric sensors
contact switches
bar code readers
pushbutton switches
Signals to output devices such as:
motors
solenoids
indicators
heaters
relays & contactors
Ladder Logic:
Sets outputs based on
input conditions
Program
Scan
Data T able
Purpose of this Appendix
Program Scan: How the
Processor Scans a
Program