GLOSSARY OF TERMS
SNAP PAC Redundancy Option User’s Guide
32
Sequential Programming
The method of programming used in a PAC Control chart. A sequentially programmed chart starts at
one block and proceeds sequentially through command blocks to the end using variables, inputs,
and outputs.
Sync Block
A block type in PAC Control used only for redundant strategies. When a sync block is encountered in
a chart, the data generated by the chart is synchronized in both the active and backup controllers.
The sync block also indicates to the backup controller what state the chart is in
(running/stopped/paused) and where in the logic to begin in case the active controller fails. A chart
that uses sync blocks is called a
transactional chart
(see the glossary entry below).
When a sync block is placed in a chart, this tells the controller not to do any writes (in that chart) to
any I/O or persistent/redundant variables until it encounters a sync block. All the various kinds of
writes that occur in a chart after a sync block are not executed until the next sync block is
encountered. In this way both controllers know what points/variables have been written to.
Sync block
For more information, see
“What Causes Synchronization to Occur” on page
.
Transaction
While the term “transaction,” is borrowed from the transactional database world, for our purposes, a
transaction is a collection of redundant operations that are logged and deferred until a
is
encountered, at which point the data generated by a transactional chart is synchronized in both the
active and backup controllers. Each transaction starts with the first write of a persistent/redundant
variable or I/O, and it ends with a sync block.
Transactional Chart
A special PAC Control chart used to support persistent/redundant data with redundant controllers. A
transactional chart always includes at least one sync block, which is used to define a
transaction
(see
