Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 82)

Chapter
5
5-1
Programming Techniques
This chapter describes techniques you can use to program a PLC-3 backup
system. In this chapter we describe:
input conditioning and timing requirements so switchover from primary
to backup processor occurs as smoothly as possible
recommendations for forcing I/O, executing block transfers, using status
bits, and reducing backup communication overhead
special considerations that apply when using communication adapter
and memory communication modules
To ensure that all rungs in the ladder program that read inputs read them
correctly, each input must remain in a steady state for at least the sum of:
(input module delay time) + (I/O scan time) + (program scan time)
This is the minimum recommended length of time during which an input
should not change state. Any input having a pulse of shorter duration may
be missed by the primary, backup, or both processors.
There are several ways you can reduce the occurrence of missed inputs,
including:
use external circuitry to keep inputs in one state for the recommended
length of time
repeat rungs that read troublesome inputs as often as necessary in the
ladder program to ensure accurate reads
use the real-time interrupt routine to scan a selected area of the program
on a real-time basis.
Chapter Objectives
Signal Conditioning