Timer and Counter Instructions
Chapter 5
55
The Timer Off-Delay instruction (TOF) can be used to turn a device on
or off after a timed interval (Figure 5.4). Like the other timer instructions,
the TOF instruction counts time-base intervals and stores this count in its
accumulated value. The TOF instruction, however, varies from the other
instructions in significant ways.
The Timer Off-Delay instruction begins to time an interval as soon as its
rung conditions go false. The enable bit, bit 17, goes false when the timer
begins (rung 1). As long as its rung conditions remain false, the TOF
continues to time, until the accumulated value equals the preset value.
When the TOF times out, bit 15 is set to zero (off) which turns off the
output (rung 2). As the rung conditions go true, bit 15 is set on and the
accumulated value is reset to 000.
Bit 17, the enabled bit, is controlled by the logic continuity of the rung.
When the rung is true, bit 17 is set to one (on); when it is false, bit 17 is set
to zero (off).
5.1.2
Timer OffDelay Instruction
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