Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 86)

Chapter
6
6-1
Switchover Considerations
When planning programs for the PLC-5 backup system, you must first
consider that the program scans of the two processors are not
synchronized; that is, the program in the primary processor is not
executing the exact same instruction at the same time as the program in the
secondary processor. In addition, you must consider:
timing
divergence
I/O
forces
Data Highway Plus switching
remote I/O switching
special sections of the data table
data
integrity
This chapter describes these switchover considerations. This chapter also
describes switchover diagnostic. It describes how to detect a possible fault
in the secondary module and how to consequently avoid switchover
problems in the backup system.
Although switchover from one processor to the other is fast — less than 50
ms, it does not happen instantly. Make note also that the total switchover
time is a factor of the 1785-BCM module delay, plus, at worst case, one
program scan of the secondary processor. The time of one program scan is
added because the secondary processor technically has not assumed control
until one scan of its program and remote I/O after the switchover.
When a failure occurs in the primary system, the remote outputs remain in
the state set by the primary processor prior to switchover, as well as while
the secondary processor assumes control of the process. Keeping this in
mind, you must program the backup system to account for the following
time requirements:
input signal update time
time-out on remote I/O link
data table transfer time over the HSSL
Chapter Objectives
Timing Requirements