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Chapter 3. Program Data
3.5 Transitions and Overrides
The %I, %Q, %M, and %G user references, and symbolic variables of type BOOL, have associated
transition and override bits. %T, %S, %SA, %SB, and %SC references have transition bits but not
override bits. The CPU uses transition bits for counters, transition contacts, and transitional coils.
Note that counters do not use the same kind of transition bits as contacts and coils. Transition bits
for counters are stored within the locating reference.
The transition bit for a reference tells whether the most recent value (ON, OFF) written to the
reference is the same as the previous value of the reference. Therefore when a reference is written
and its new value is the same as its previous value, its transition bit is turned OFF. When its new
value is different from its previous value, its transition bit is turned ON. The transition bit for a
reference is affected every time the reference is written to. The source of the write is immaterial; it
can result from a coil execution, an executed function's output, the updating of reference memory
after an input scan, etc.
When override bits are set, the associated references cannot be changed from the program or the
input device; they can only be changed on command from the programmer. Overrides do not protect
transition bits. If an attempted write occurs to an overridden memory location, the corresponding
transition bit is cleared.
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PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
GFK-2950C
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