User guide

A–B
17
Chapter
17–1
Timer and Counter Instructions
This chapter covers the following timer and counter instructions for use with
all processors except where noted:
• Timer On-Delay (TON)
• Timer Off-Delay (TOF)
• Retentive Timer On-Delay (RTO)
• Count Up (CTU)
• Count Down (CTD)
• High–Speed Counter (HSC) – fixed controller only
• Counter or Timer Reset (RES)
Timers and counters are output instructions. They include:
• Timer On-Delay (TON). It counts timebase intervals when the rung is
true and resets when the rung is false (non–retentive). The timebase is
selectable as 0.01 sec or 1.0 sec for SLC 5/02 processors, and set at 0.01
sec for fixed controllers and SLC 5/01 processors. See page 17–3.
• Timer Off-Delay (TOF). It counts timebase intervals when the rung is
false and resets when the rung is true (non–retentive). The timebase is
selectable as 0.01 sec or 1.0 sec for SLC 5/02 processors, and set at 0.01
sec for fixed controllers and SLC 5/01 processors. See page 17–4.
• Retentive Timer On-Delay (RTO). An on-delay timer which retains its
accumulated value when the rung goes false. See page 17–5.
• Count Up (CTU). The count increments at each false-true transition of
the rung. See page 17–7.
• Count Down (CTD). The count decrements at each false-true transition
of the rung. See page 17–7.
• High–Speed Counter (HSC). A special CTU counter for use with fixed
controllers having 24 VDC inputs. See page 17–9.
• Counter or Timer Reset (RES). This instruction resets the accumulated
value and status bits of a counter or timer. It cannot be used with TOF
timers. See page 17–13
.
Timer and counter instructions have 3-word data file elements, illustrated on
pages 17–2 and 17–7. Word 0 is the control word, containing the status bits
of the instruction. Word 1 is the preset value. Word 2 is the accumulated
value.
The accumulated value is the current number of timebase intervals that have
been measured for a timer instruction; for a counter instruction, it is the
number of false-to–true transitions that have occurred. The preset value is
the set point that you enter in the timer or counter instruction.
When the accumulated value becomes equal to or greater than the preset
value, the done status bit is set. You can use this bit to control an output
device.
Timer and Counter
Instructions Overview