Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 160)

Publication 1747-UM010B-EN-P - September 2003
9-18 Programming Techniques
Timer Instructions
The SLC 500 processors maintain timers by keeping a copy of a
hardware timer in a portion of the three-word structure (timer byte)
used by each timer.
The hardware timer of the primary processor is completely
asynchronous to the hardware timer of the secondary processor. If the
whole three-word timer structure, including the timer-byte, is
transferred from the primary to the secondary processor during a
switchover, then the timer could encounter a large positive increment
in the timer accumulated value. The worst-case increment may be as
much as 2.55 seconds.
Follow the guidelines below to help avoid this type of increment in
the timer accumulated value:
•
Do not send the first word of the timer control structure.
•
Transfer only the accumulated value. (Transfer the preset value
also, if you need to change it.)
Even though both processors are executing the same programs, your
program should transfer the timer accumulated value from the primary
processor to the secondary processor at least once after you:
•
start the backup system
•
switch the secondary SLC 500 from PROGRAM to RUN mode
•
restart a repaired system
This allows the accumulated value in the secondary processor to track
the accumulated value in the primary processor.
Control byte Timer byte 1
Preset value
Accumulated value
2
Accumulated value 3