Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 83)

Programming Techniques
Chapter 5
5-2
Using Repetition to Handle Short Input Pulses
One method of handling short input pulses is to repeat the rungs that read
those inputs. For example, if the duration of an input pulse is only 70% of
the recommended time, you could repeat rungs depending on this input at
least twice in the program. Put approximately one third of the program
between the rungs.
Using Interrupts to Handle Short Input Pulses
Another method of handling short input pulses is to use a real-time
interrupt routine to scan a selected area of the program on a real-time basis.
For further information about this technique, refer to the PLC-3 Family
Programmable Controller Programming Reference Manual (publication
1775-6.4.1).
Special consideration must be given when timers and counters are being
transferred from the primary processor to the backup processor. Both
timers and counters consist of three words. The memory communication
module transfers only one word at a time and a switchover could occur
before all three words have been transferred.
One of the timer words contains the timer accumulator and another word
contains the information used to maintain the accuracy of the timer.
Because the primary and backup hardware are asynchronous, at switchover
the accumulator may be increased to a larger value than has actually
elapsed. Timers having a time base of:
10 milliseconds may be off by as much as +150 milliseconds
100 milliseconds may be off by as much as +300 milliseconds
1 second may be off by as much as +1.5 seconds
In a similar manner, the possibility exists that the counter accumulator may
increment twice at switchover.
Timers and Counters at
Switchover