OPTO 22 SNAP PAC Redundancy Option, User Manual

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CREATING A REDUNDANT STRATEGY
SNAP PAC Redundancy Option User’s Guide
44
• Construct logic using sequential programming. See “Transaction Example” on page 43 .
• Do not use long delays.
When developing a transactional chart, keep the following constraints in mind:
• Counters cannot be cleared.
• Latches are unavailable (but a counter can be used in place of a latch by storing and comparing
count values).
• Min/Max clearing is not supported, so Min/Max functionality is not useful.
• The following commands are not supported in redundant strategies:
– Call Chart
– Clear All Latches
– Clear Counter
– Clear Off-Latch
– Clear On-Latch
– Continue Calling Chart
– Get & Clear Analog Maximum Value
– Get & Clear Analog Minimum Value
– Get & Clear Counter
– Get & Clear Off-Latch
– Get & Clear On-Latch
– Get Off-Latch
– Get On-Latch
– IVAL Set Off-Latch
– IVAL Set On-Latch
• Subroutines may not contain sync blocks. However, a subroutine may perform operations on
redundant objects.
• Called charts are not supported. (Call Chart is not supported, but Start Chart is.)
• You cannot do background downloads. However, PAC Redundancy Manager allows you to
download to the non-active controller and then quickly switch over to it.
Using Reads and Timers
Reads can occur at any time. Reading an analog channel, for example, occurs the moment the
command is executed in the chart. The same is true when reading any kind of variable or change
to I/O.
Timers are similar to redundant variables in that if you start a timer, it does not start in the other
controller until a sync block is encountered; starting a timer is a write operation. Once started and
synced, the timer is activated in both controllers. Each controller can read from that timer without
requiring a sync block or synchronization to occur. However, stopping a timer requires a sync block
to be encountered to stop the timer on both controllers.