Chapter 3. Program Data42 PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual GFK-2950C3.5 Transitions and OverridesThe %I, %Q, %M, and %G user references, and symbolic variables of type BOOL, have associatedtransition and override bits. %T, %S, %SA, %SB, and %SC references have transition bits but notoverride 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 bitsfor counters are stored within the locating reference.The transition bit for a reference tells whether the most recent value (ON, OFF) written to thereference is the same as the previous value of the reference. Therefore when a reference is writtenand its new value is the same as its previous value, its transition bit is turned OFF. When its newvalue is different from its previous value, its transition bit is turned ON. The transition bit for areference is affected every time the reference is written to. The source of the write is immaterial; itcan result from a coil execution, an executed function’s output, the updating of reference memoryafter an input scan, etc.When override bits are set, the associated references cannot be changed from the program or theinput device; they can only be changed on command from the programmer. Overrides do not protecttransition bits. If an attempted write occurs to an overridden memory location, the correspondingtransition bit is cleared.
