CHAPTER 4: CREATING A STRATEGY WITH CONTROLLER REDUNDANCY
SNAP PAC Redundancy Option User’s Guide
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The chart restarts on the new active controller beginning with the closest sync block previous to the
failure point.
2)
Chart starts here on
the backup controller
1)
Active controller
fails here
Active Backup
Transaction Example
In this example of a transactional chart, a cookie baking process is implemented on redundant
controllers using sequential programming. As shown in the illustration, the highlighted transaction
starts when the conveyor is stopped. It continues by turning on the oven and the bake timer. The
transaction ends at the next sync block.
Transaction
starts
Transaction ends
Sync block
Short delay
Using a redundant bake timer variable maintains the proper baking time. If a long delay were used
instead to control bake time, a restart to the previous sync block would result in the delay occurring
twice and the cookies would burn. Short delays can be used to facilitate the efficient execution of a
strategy. However, longer delays should not be used in a transactional chart.
Design Considerations and Chart Constraints
Consider the following guidelines when designing a redundant control strategy:
•
Use persistent/redundant variables only as necessary. Because each additional
persistent/redundant variable must be synchronized, too many persistent/redundant variables
can increase system overhead and slow down the system.
