Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 107)

Programming Techniques
Chapter 7
7-10
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 this 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 up to 2.55 seconds, which may cause a large bump.
To help prevent this from occurring:
do not send the first word of the timer control structure
transfer only the accumulated value, and possibly the preset value (if you
have the 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 up the backup system
switch the secondary PLC-5 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.
Counter Instructions
Counter instructions are similar in structure to timer instructions; however, there
is only one problem that could exist with these instructions when used in a
PLC-5 backup system. At the point of switchover, a counter that occurs once in
a program could increment or decrement twice in one program scan. This
problem arises when the following sequence of events occur:
1. The primary system executes a counter up/down instruction conditioned
by a rung transition of not-true to true. This increments/decrements the
counter accumulated value.
2. The 1785-BCM modules transfer this data into the counter file of the
backup processor before the secondary’s input image table is updated with
the input transition information.
3. The backup processor updates its input image table.
4. The processor in the secondary system (which was at a different point in
its program scan) executes the counter instruction again, thus
incrementing/decrementing the counter again.