Prior to programmable logic controllers, timing functions in industrial automation were implemented using various mechanical and electromechanical timers. Programmable logic controllers allowed timing functions to be programmed digitally, eliminating the need for external timers. A PLC timing instruction counts intervals of a selectable time base and uses bits to indicate when timing conditions are met. Standard timing functions like on delay, off delay, interval, retentive and symmetrical/asymmetrical recycle can be created using combinations of timing instructions, and cascading timers allows complex sequences.

1. IEC2102
By Brian Khairullah

2. • Prior to the introduction of Programmable controllers in
the late 1960s timing functions were implemented by the
use of a variety of timers that included:
• dashpot (pneumatic, hydraulic)
• synchronous clock
• motor driven
• capacitor time limit relays
• electronic (solid-state).
• (Herman, 2014, p. 85 -88; Rockis & Mazur, 2014, p. 347-349

3. • ON Delay – When the timer coil is energized there is a delay before the
contacts change position. The contacts remain in the changed position until
the timer coil is de-energized at which time the contacts return to the normal
position immediately.

4. • OFF Delay – When the timer coil is energized the contacts change position
immediately and remain in the changed position. When the timer coil is de-
energized there is a delay before the contacts return to the normal position.

5. • Interval (One-shot) – When the timer coil is energized the contacts change
immediately and remain in the activated position for the duration of the
preset time period after which they return to their normal position.
• Retentive Timer: If power is removed from the timer coil during its timing
phase it will retain the accumulated time and when power is restored to the
coil it will begin timing at the point that it stopped.
• Recycle – When the timer is energized the contacts change position at a
rate determined by the preset time.
• Symmetrical Recycle: The ON time and OFF time are equal.
• Asymmetrical Recycle: The ON time and OFF time are not equal.

6. • An advantage of a PLC is that timing functions can be
programmed eliminating the need for external timers.
• All timing functions can be duplicated in a PLC.

7. • Timer: Identifies the timer and the location of the timer
information. File type T4 followed by the timer number
which can be a value from 0 to 99. e.g. T4:6
• Time Base: The duration of timebase interval that time is
incremented in, in seconds. Can be 0.1 or 0.01 so a
value of 100 would equal 10 seconds with a time base of
0.1 or 1 second with time base of 0.01.
• Preset: Determines the amount of the time that must
pass to activate the done (DN) bit. Expressed in units of
the time base.

8. • Accumulator – Accum: While the timer is in timing
mode (timing function activated) the accumulator
increments in units of the time base.
• Enable Bit - EN: True when the timer input is true.
• Done Bit - DN: True when the accumulated time equals
the preset time.
• Timer Timing Bit - TT: True while the accumulator is
incrementing.

9. • Counts timebase intervals when the instruction is true
(Allen-Bradley, 2008, p. 2-9)
• When the timer input is activated there is a delay, equal to the
preset time, before the done bit (DN) changes from zero to one.
i.e. DN is true when the accumulated time equals the preset time.
• When the timer input is deactivated the done bit changes back to
zero immediately.

10. • Counts timebase intervals when the instruction is false
(Allen-Bradley, 208, p. 2-10).
• When the timer input is activated the done bit (DN) changes from
zero to one immediately. When the timer input is deactivated the
accumulator begins incrementing and there is a delay, equal to the
preset time, before the done bit changes back to zero. i.e. DN is
true until the accumulated time equals the preset time.

11. • Counts timebase intervals when the instruction is true
and retains the accumulated value when the instruction
goes false or when power cycle occurs(Allen-Bradley,
208, p. 2-11).
• RTO is an On Delay that retains its accumulated value if the input
is deactivated while it is incrementing and then begins timing again
at the same point once the input is reactivated.
• The Reset instruction resets the accumulated value to zero.

12. • Other timing functions can be created using these timers
in various combinations with each other and with other
contacts:
• Symmetrical Recycle: An XIO contact in series with the input of a
timer controlled by the done bit of that timer will create a recycle
timer by “self-resetting” the timer.

13. • Asymmetrical Recycle: A self-resetting recycle timer that controls
an output with the addition of a comparative operator in series with
the output. On time and Off time can be different values.
• In this example for each 1 second cycle Lamp-3 will be off for
0.7 seconds and on for 0.3 seconds.

14. • Cascading Timers: The done bit of one timer activates the input of
another timer. Multiple timers can be used in a “cascade.”
• A self-resetting cascade can be created by the addition of an XIO
controlled by the done bit of the last timer in the cascade placed in
series before the first timer.

15. • Interval: Here is one way to create an interval timer.

16. • A variety of options
• Reliable precision
• Flexibility for any application

17. • Allen-Bradley. (2008). SLC 500 instruction set reference
manual. Rockwell Automation. Retrieved from
http://literature.rockwellautomation.com/idc/groups/literat
ure/documents/rm/1747-rm001_-en-p.pdf
• Herman, S. L. (2014). Industrial motor control 7th Ed. l.
Clifton Park, NY: Cengage Learning.
• Rockis, G. J. & Mazur, G. A. (2014). Electrical motor
controls for integrated systems 5th Ed. Orland Park, IL:
American Technical Publishers
• Rockwell Automation. Retrieved from
http://ab.rockwellautomation.com/

18. • Simpson, B. (2012). The learning pit dot com. Retrieved
from http://www.thelearningpit.com/