Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 118)

Chapter
9
9-1
Calculating Program Timing
This chapter provides information to help you determine the program
timing for your PLC-5 programmable controller system.
If you want to read about: Go to
page:
PLCĆ5 processor scan time 9Ć1
I/O update timing:
Transfer discrete data
Transfer block data 9Ć5
9Ć5
Instruction timing and
memory requirements:
Bit and word instructions
File instructions
9Ć7
9-8
9Ć11
Program constants 9Ć13
Direct and indirect elements 9Ć15
The basic function of a programmable controller system is to read the
status of various input devices (such as pushbuttons and limit switches),
make decisions based on the status of those devices, and set the status of
output devices (such as lights, motors and heating coils). To accomplish
this, the PLC-5 processor performs two primary operations:
program scanning—where
- logic is executed
- housekeeping is performed
I/O scanning—where input data is read and output levels are set
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. You
can monitor the program scan time using the processor status screen.
Housekeeping activities for most PLC-5 processors includes:
Chapter Objectives
Introduction to Classic
PLCĆ5 Processor Scanning
Housekeeping
Logic Scan
System Design
Determined
Choosing
Communication
Transferring Discrete
and Block Data
Planning Your
System Programs
Selecting Interrupt
Routines
Calculating Program
Timing
Choosing Hardware
Placing System
Hardware
Assigning Addressing
Mode, Racks,
and Groups