Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 106)

Programming Techniques
Chapter 7
7-9
Some instructions, when used with a PLC-5 backup system, may operate
unpredictably when a switchover occurs if you fail to observe certain
programming considerations. For example, you must consider transferring the
control element and data elements for instructions that are also being executed
in the secondary processor. You also must consider transferring data files which
contain more than one word.
Instructions we discuss in this section include:
timer
counter
logical, arithmetic, compare and move
diagnostic, sequencing, file arithmetic and logic, file search and compare, file
copy and fill
FIFO and bit displacement
block
transfer
message
In the remainder of this chapter, we describe the behavior of these instructions
in the PLC-5 backup system.
Important: You must be aware when programming your backup system that
the primary and secondary system’s program scans are not synchronized; that is,
the systems do not necessarily execute the same instructions at the same time.
You must also consider that the PLC-5 processor executes block transfers
asynchronously to the program scan, and therefore, interrupts this scan
asynchronously to access block transfer write (BTW) and block transfer read
(BTR) files.
Timer Instructions
The PLC-5 family 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
(Figure 7.4).
Figure 7.4
Timer Structure
Control byte Timer byte
Preset value
Accumulated value
Word
1
2
3
11111I
Accounting for Instructions
That Could Cause Problems
During Switchover