Page 153 - Programmable Logic Controllers, Fifth Edition

L1
S1 L2 Timing diagram TON TOF RTO CTU CTD RES HSC
HSC
TD 10 s
10 s User Bit Timer/Counter Input/Output Compare
L1
TD1 Input Figure 7-8 Timer selection toolbar.
Output On
O
Figure 7-7 Off-delay timer circuit that uses a normally TOF (Timer Off Delay)—Counts time-based inter-
closed, timed closed (NCTC) contact. vals when the instruction transitions from a true to
false condition.
Figure 7-7 shows an off-delay timer circuit that uses a RTO (Retentive Timer On)—Counts time-based
normally closed, timed closed (NCTC) contact. The op- intervals when the instruction is true and retains the
eration of the circuit can be summarized as follows: accumulated value when the instruction goes false or
when power cycle occurs.
• With S1 initially open, TD coil is de-energized so
TD1 contacts are closed and light L1 will be on. RES (Reset)—Resets a retentive timer’s accumulated
• When S1 is closed, TD coil is energized and TD1 value to zero.
contacts open instantly to switch light L1 off. Several quantities are associated with the timer
• When S1 is opened, TD coil is de-energized and the instruction:
timing period starts. TD1 contacts are delayed from • Time Base The time base of a timer is the unit
closing so L1 remains off. of time used by a timer to time an event. A timer
• After the 10 s time-delay period has elapsed, TD1 instruction times an event by counting the number
contacts close to switch the light on. of times the time base has occurred. Depending on
the manufacturer and type of PLC, time base values
7.2 Timer Instructions can be in 1 ms (0.001 s), 10 ms (0.01 s), 100 ms
(0.1 s), or 1 second intervals. For example, if a timer
PLC timers are instructions that provide the same func- has a time base of 1 second and it is timing some-
tions as on-delay and off-delay mechanical and electronic thing that is 5 seconds long, the PLC will wait until
timing relays. All PLC timers are output instructions. PLC the time base has occurred 5 times before the timer
timers offer several advantages over their mechanical and times out. Conversely, if the PLC’s time base setting
electronic counterparts. These include the fact that: is 0.01 seconds, it will wait until the time base has
• The entire timing function occurs inside the controller. occurred 500 times before timing out. The smaller
• Time settings can be easily changed. the time base, the better the accuracy of the timer.
• The number of timers used in a circuit can be in- • Preset Value The preset value of a timer repre-
creased or decreased through the use of program- sents the time duration for the timing circuit. Total
ming changes rather than wiring changes. timing interval = the preset value x time base. For
example, for a timer with a preset value of 100 and
• Timer accuracy and repeatability are extremely a time base of 0.1s the time duration for the timer is:
high because time delays are generated in the PLC Total timing interval = 100 × 0.1 s
processor. = 10 seconds

In general, there are three different PLC timer types: the • Accumulated Value The accumulated value of
on-delay timer (TON), off-delay timer (TOF), and retentive the timer represents the amount of time that has
timer on (RTO). The most common is the on-delay timer, elapsed from the moment the timing started. It
which is the basic function. There are also many other tim- keeps track of how many times the time base has
ing configurations, all of which can be derived from one or occurred since the timer instruction was initiated.
more of the basic time-delay functions. Figure 7-8 shows
the timer selection toolbar for the Allen-Bradley SLC 500 Although each manufacturer may represent timers
PLC and its associated RSLogix software. These timer differently on the ladder logic program, most timers operate
commands can be summarized as follows: in a similar manner. One of the first methods used depicts
the timer instruction as a relay coil similar to that of a me-
TON (Timer On Delay)—Counts time-based inter- chanical timing relay. Figure 7-9 shows a coil- formatted
vals when the instruction is true. timer instruction.

134 Chapter 7 Programming Timers

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