1. Programmable Logic Controllers
Third Edition
Frank D. Petruzella
McGraw-Hill

2. Chapter 7
Programming
Timers

3. Timers
There are very few industrial control systems that do not need at least one or two timed functions. They are used to
activate or de-activate a device after a preset interval of
time.
Time delay relays and solid-state timers are used to provide a time delay. They may have displays, pots or other means of operator interface for time settings and electromechanical or solid state outputs.
Time Delay
Relay
Solid-State
Timer

4. On-Delay Timing Relay Operating coil
Instantaneous
contacts NO NC
Nontimed contacts are controlled directly by the timer coil, as in a general-purpose control relay.
Time control
contacts NO NC
Time adjustment
When the coil is energized, the timed contacts are are prevented from opening or closing until the time delay period has elapsed. However, when the coil is de-energized, the timed contacts return instantaneously to their normal state.

5. Timed Contact Symbols On-Delay Symbols
Normally open, timed closed contact (NOTC)
Contact is open when relay coil is de-energized
When relay is energized, there is a time delay in closing
Normally closed, timed open contact (NCTO)
Contact is closed when relay coil is de-energized
When relay is energized, there is a time delay in opening

6. Timed Contact Symbols Off Delay Symbols
Normally open, timed open contacts (NOTO).
Contact is normally
open when relay coil
is de-energized.
When relay coil is energized, contact closes instantly.
When relay coil is de-energized, there is a time delay before the contact opens.
Normally closed, timed closed contacts (NCTC).
Contact is normally
closed when relay coil
is de-energized.
When relay coil is energized, contact opens instantly.
When relay coil is de-energized, there is a time delay before the contact closes.

7. On-Delay Relay Timer Circuit (NOTC Contact)
Sequence of operation S1
S1 is opened, TD de-energizes, TD1 opens instantly, L1 is switched off.
S1 closes, TD energizes,
timing period starts,
TD1 still open, L1 is still off.
S1 open, TD de-energized,
TD1 open, L1 is off. L1
After 10 s, TD1 closes, L1 is switched on.
10 s
10 s
OFF ON
Input
Output
Timing Diagram

8. On-Delay Relay Timer Circuit (NCTO Contact)S1
Sequence of operation
S1 closes, TD energizes,
timing period starts, TD1
is still closed, L1 is still on.
S1 is opened, TD de-energizes,
TD1 closes instantly, L1 is
switched on.
S1 open, TD de-energized,
TD1 closed, L1 is on. L1
After 10 s, TD1 opens,
L1 is switched off.
10 s
10 s On
Off
Input
Output
Timing Diagram

9. Off-Delay Relay Timer Circuit (NOTO Contact)
Sequence of operation S1
S1 is opened, TD de-energizes,
timing period starts, TD1 is
still closed, L1 is still on.
S1 closes, TD energizes,
TD1 closes instantly,
L1 is switched on.
S1 open, TD de-energized,
TD1 open, L1 is off. L1
After 10 s, TD1 opens, L1 is
switched off.
10 s
10 s
Input
Output
Off On
Timing Diagram

10. Off-Delay Relay Timer Circuit (NCTC Contact)
Sequence of operation S1
S1 is opened,
TD de-energizes, timing
period starts, TD1 is still
open, L1 is still off.
S1 closes, TD energizes,
TD1 opens instantly,
L1 is switched off.
S1 open, TD de-energized,
TD1 closed, L1 is on.
After 10 s, TD1 closes,
L1 is switched on. L1
10 s
10 s
Input
Output On
Off
Timing Diagram

11. 1. Timers are used to activate or de-activate a
device after a preset interval of time.
(True/False)
2. With most timers the time delay period is fixed
and can not be varied. (True/False)
3. ________ contacts are controlled directly by the
timer coil, as in a general-purpose control relay.
timed (c) instantaneous
NO (d) NC

12. 4. When a relay timer coil is de-energized, the timed contacts return instantaneously to their normal state. (True/False)
5. Which of the following symbols represents a
normally open timed closed contact?

13. 6. Which of the following symbols represents a
normally closed timed open contact?

14. 7. The timed relay contact shown is designed to operate so that:
a. when the relay coil is energized, there is a
time delay in closing
b. when the relay coil is energized, there is a
time delay in opening
c. when the relay coil is de-energized, there is a
d. when the relay coil is de-energized, there is a

15. 8. The timing diagram shown is that of an
a. on-delay timer circuit (NOTC contact)
b. on-delay timer circuit (NCTO contact)
c. off-delay timer circuit (NCTC contact)
d. off-delay timer circuit (NOTO contact)
10 s
Input
Output
Off On
Timing Diagram

16. 9. In the circuit shown, the light will stay off
a. as long as S1 is closed
b. for 10 seconds after coil TD is energized
c. for 10 seconds after coil TD is de-energized
d. both a and c L1 L2 S1
10 s

17. 10. In the circuit shown, the light will stay on
as long as S1 is closed
as long as S1 is open
c. for 10 seconds after S1 is closed
d. both b and c

18. Programmed Timer Instructions
PLC timers are output instructions that provide the same functions as timing relays and solid state timers.
Some advantages of PLC timers:
their settings can be altered easily
the number of PLC timers used
can be increased or decreased by
programming changes without
wiring changes
timer accuracy and repeatability
are extremely high

19. RSLogic Timer Commands
Timer/Counter
TON
Command Name Description
Timer On-Delay
Counts time base
intervals when the
instruction is “true”

20. RSLogic Timer Commands
Timer/Counter
Command Name Description
TOF
Timer Off-Delay
Counts time base
intervals when the
instruction is “false”

21. RTO RSLogic Timer Commands Command Name Description Retentive Timer ON
Counts time base intervals when the instruction is “true” and retains the accumulated value when the instruction goes "false" or when power cycle occurs
Timer/Counter
RES
Reset
When this instruction is "true" it resets the count of the RTO counter

22. Quantities Associated with the Timer Instruction
Preset Time – Represents the time duration of the timing
circuit. For example, if a time delay of 10 s is required, the timer will have a preset of 10 s.
Accumulated Time – Represents the amount of time that
has elapsed from the moment the timing coil became energized.
Time Base – Timers can typically be programmed with
several different time bases: 1 s, 0.1 s, and 0.01 s are typical time bases. For example, if you enter 0.1 for the time base and 50 for the preset time the timer would have a 5 s delay (50 x 0.1 s = 5 s).

23. Coil-Formatted Timer Instruction
Contact determines
rung continuity
The timer assigned
an address
XXX
TON
The type of timer
is specified
Preset value PR:YYY
Time base s
Accumulated value AC:000
When the timer rung has logic continuity, the timer's accumulated value increases. When accumulated value equals the preset value, the output is energized and and the timed output contact associated with the output is closed. The timed contact can be used as many times as you wish throughout the program as a NO or NC contact.

24. Generic Block-Formatted Timer Instruction
Timers are most often represented by boxes in a ladder logic.
Control line controls the actual timing operation of the timer.
Whenever this line is true the timer will time.
Preset time
Time base
Accumulated time
Retentive timer block
Output line
The timer continuously compares its accumulated time with its preset time. Its output is logic 0 as long as the accumulated time is less than the preset time. When the two become equal the output changes to logic 1.
Reset line resets the the timer's accumulated value to zero.

25. On-Delay Timer Instruction
The on-delay timer operates so that, when the rung containing the timer is true, the timer time-out period commences.
Timer
Input
Rung condition
Timed period
False
True
On delay
time duration
Timed output bit
OFF ON
Timer Sequence
The timed output becomes true sometimes after the timer rung becomes true; hence the timer is said to have an on delay.

26. Allen-Bradley On-Delay Timer Instruction
Allen-Bradley PLC-5 and SLC-500 controller timer elements each take three data table words: the control word, preset word, and accumulated word.
The control word uses three control bits: Enable (EN) bit,
Timer-Timing (TT) bit, and Done-Bit (DN).
TIMER TABLE
T4:0
/EN
/TT
/DN

27. Allen-Bradley On-Delay Timer Instruction
The Enable (EN) bit is true (has a status of 1) whenever the timer instruction is true. When the timer instruction is false, the enable bit is false (has a status of 0)
Enable bit false
TON
TIMER ON DELAY
Timer T4:0 EN
T4:0
Enable bit true

28. Allen-Bradley On-Delay Timer Instruction
The Timer-Timing (TT) bit is true whenever the accumulated value of the timer is changing, which means the timer is timing.
TIMER ON DELAY
TON
Timer T4:0
Preset
Accumulated EN TT
T4:0
Timer-Timing bit true

29. Allen-Bradley On-Delay Timer Instruction
The Done-Bit (DN) changes state whenever the accumulated value reaches the preset value. Its state depends on the type of timer being used.
TON
TIMER ON DELAY EN
Timer T4:0
Preset
Accumulated DN
Done-bit changes state 50
T4:0 DN

30. Allen-Bradley On-Delay Timer Instruction
The preset value (PRE) word is the set point of the timer, that is, the value up to which the timer will time.
The accumulated value (ACC) word is the value that increments as the timer is timing. The accumulated value will stop incrementing when its value reaches the preset value.
TIMER TABLE
T4:0
/EN
/TT
/DN
.PRE
.ACC

31. Allen-Bradley On-Delay Timer Instruction
The information to be entered includes:
TON
TIMER ON DELAY EN DN
Timer T4:0
Timer number which must come from the timer file.
Time base
Time base which is expressed in seconds.
Preset
Preset value which is the length of the time delay.
Accumulated
Accumulated value which is normally entered as 0.

32. On-Delay Timer Program
Ladder Logic Program L1 L2
TON
Input A
TIMER ON DELAY
Timer T4:0
Time base
Preset
Accumulated
Input A
Output B G EN DN 10
Output C R
T4:0
Output B EN
Output D Y
T4:0
Output C TT
T4:0
Output D DN

33. On-Delay Timer Program
Timing Diagram
Input condition A On
Off
Timer-enable bit
Timer-timing bit
Timer-done bit
Timer accumulated
value
4 s

34. On-Delay Timer Program
Timers are 3-word elements
Word 1 2
Word 0 is the control word EN TT DN
Internal use
Word 1 stores the preset value
Preset value PRE
Word 2 stores the accumulated value
Accumulated value ACC

35. On-Delay Timer With Instantaneous Output
Relay Ladder Schematic Diagram L1 L2
Stop
Start
1TD M
1TD-1
(instantaneous
contact)
1TD-2
(5 s)
(timed contact)
1TD

36. On-Delay Timer With Instantaneous Output
Programmed Circuit
Motor M
Internal
relay
Stop
Start
Timer
PR: 5
TB: 1 s
Output
line L1 L2
Inputs
Ladder logic program

37. Start-Up Warning Signal Circuit
Relay Ladder Schematic Diagram L1 L2
Start-up
PB1
Reset
PB2
CR1
CR1-1
CR1-2
1TD-1
(10 s)
CR1-3
1TD
Horn

38. Start-Up Warning Signal Circuit
TON
TIMER ON DELAY
Timer T4:0
Time base
Preset
Accumulated
Inputs
Output
Ladder logic program
Programmed Circuit
Horn
Start-up
Reset
PB1
PB2 EN DN
T4:0 10

39. Timed Closed Solenoid Value Program
Input L1
Switch
SW_1
Ladder logic program
Output L2
Valve EN DN
timer_1.dn
TON
Timer On Delay
Timer timer_1
Preset
Accumulated
12000

40. Automatic Sequential Control System
Stop
PB1
Start
PB2
Lube oil
pump motor OL M1
M1-1
PS1
(lube oil
pressure switch)
Main drive
motor M2
1TD
1TD-1
(15 s)
Feed M3
Relay Ladder
Schematic
Diagram

41. Automatic Sequential Control System
Programmed Circuit
Ladder logic program
Inputs
PB1
PB2 M1
Outputs OL
PB1 M1 M1
PB2 OL M2
PS1
PS1 M2 OL 15 M3
TON
TIMER ON DELAY
Timer T4:0
Time base
Preset
Accumulated EN DN
T4:0 M3 DN

42. Off-Delay Programmed Timer
The off-delay timer (TOF) operation will keep the output energized for a period after the rung containing the timer has gone false. EN DN
TOF
TIMER OFF DELAY
TIMER T4:3
Time base
Preset
Accumulated
I:1.0/0
O:2.0/1
T4:3/DN PL L1 L2
Input
Output
Ladder logic program S1 15

43. Off-Delay Programmed Timer
Timing Diagram
Input condition S1
True
False
Timed period
Off delay
timed duration
Timed output
O:2.0/1
True (logic 1)
False (logic 0)
Preset value = accumulated value

44. Off-Delay Timer Used To Switch Motors Off
Input L2
Output
Ladder logic program L1
5000
10000
15000

45. Pneumatic Off-Delay Timer
Relay Ladder
Schematic
Diagram L1 L2

46. Programmed Pneumatic Off-Delay Timer
Equivalent Programmed Circuit L1 L2
Input
Outputs
Ladder logic program 5

47. Fluid Pumping Process Operation Before starting, PS1 must be closed.
When the pump start button is pressed, the pump starts. The button can then be released and the pump continues to operate.
When the stop button is pushed, the pump stops.
PS2 and PS3 must be closed for 5 s after the pump starts. If either PS2 or PS3 opens, the pump will shut off and will not not be able to start again for another 14 s.

48. Fluid Pumping Process Program
Inputs
Output
Ladder logic program 5

49. Retentive Timer
A retentive timer accumulates time whenever the device receives power, and maintains the current time should power be removed from the device. Once the device accumulates time equal to its preset value, the contacts of the device change state. The retentive timer must be intentionally reset with a separate signal for the accumulated time to be reset.
Electromechnical Retentive Timer
Cam operated
contact
Motor-driven
cam
Once power is applied, the motor starts turning the cam. The positioning of the lobes determines the time it takes to activate the contacts. If power is removed from the motor, the shaft stops but does not reset.

50. Retentive On-Delay Timer Program
The PLC-programmed RETENTIVE ON-DELAY timer (RTO) operates in the same way as the nonretentive on-delay timer (TON), with one major exception. This is a retentive timer reset (RTR) instruction.
Unlike the TON, the RTO will hold its accumulated value when the timer rung goes false and will continue timing where it left off when the timer rung goes true again. This timer must be accompanied by a timer reset (RES) instruction to reset the accumulated value of the timer to zero.
Same address

51. Retentive On-Delay Timer Program9 3

52. Retentive On-Delay Timing Chart
Enable bit is reset when input pushbutton PB1 is opened
Accum = Preset
Accum value retained
when rung goes false
When reset PB2 is closed, the T4:2/DN bit is reset to 0. Accumulated value is reset and held at zero until the reset pushbutton is opened.

53. Retentive On-delay Alarm Program
Ladder logic program L2 L1
60000
The purpose of the RTO timer is to detect whenever a piping system has sustained a cumulative overpressure condition of 60 s. At that point, a horn is sounded automatically. You can silence the alarm by switching the key switch to the rest position.

54. Bearing Lubrication Program Sequence Of Operation
To start the machine, the operator turns SW on.
Before the motor shaft starts to turn, the bearings are supplied
with oil by the pump for 10 s.
The bearings also receive oil when the machine is running.
When the operator turns SW off to stop the machine, the oil
pump continues to supply oil for 15 s.
A retentive timer is used to track the total running time of the
pump. When the total running time is 3 h, the motor is shut down
and a pilot light is turned on to indicate that the filter and oil
need to be changed.
A reset button is provided to reset the process after the filter and
oil have been changed.

55. Bearing Lubrication Program
10800 15 10

56. Cascading Timers
The programming of two or more timers together is called cascading. Timers may be interconnected, or cascaded to satisfy any required control logic.
Three motors started automatically in sequence with a 20-s time delay between each motor startup.
Relay Schematic Diagram

57. Equivalent Time-Delayed Motor-Starting Program
20000
20000

58. Annunciator Flasher Circuit
Two timers can be interconnected to form an oscillator circuit. The oscillator logic is basically a timing circuit programmed to generate periodic output pulses of any duration. They can be used as part of an annunciator system to indicate an alarm condition.
The oscillator circuit output is programmed in series with the alarm condition. If the alarm condition is true, the appropriate output indicating light will flash.

59. Annunciator Flasher Circuit

60. Cascading of Timers for Longer Time Delays
12000
30000

61. Control of Traffic Lights in One Direction
A typical application for PLC timers is the control of traffic lights.
Control of Traffic in One Direction
Sequence of Operation
Red
30 s
Amber
5 s
Green
25 s

62. Control of Traffic Lights in One Direction5 30 25

63. 11. The timed contact of a PLC timer can only
be used as a normally-open contact. (True/False)
12. The ______ bit operates the same as an instantaneous contact on a timer relay.
a. enable
b. done
c. timer-timing
d. timer number

64. 13. The preset time of a PLC timer represents the
amount of time that has elapsed from the moment
the timing coil became energized. (True/False)
14. If the preset time of a timer is 150 and the time
base is 0.1 seconds, the time-delay period would
be 1500 seconds. (True/False)

65. 15. In general, the three different types of PLC timers are:
a. TON, TOF, and PRE
b. TON, TOF, and RTO
c. TON, ACC, and RTO
d. TT, EN, and DN

66. 16. The amount of time for which a timer is
programmed is called the
a. preset
b. set point
c. Done Time (DN)
d. accumulated time

67. 17. The timer reset instruction must be addressed
to the same address as the ______ instruction.
a. TON
b. TOF
c. RTO
d. EN

68. 18. Which of the following statements is not true for a retentive on-delay timer?
a. The timer accumulates time when it is energized.
b.The timer requires a reset instruction to reset the accumulated value of the timer to zero.
c. The timer does not reset to zero when it is de-energized.
d. The reset input to the timer will not override the control input.

69. 19. The timer instruction is:
a. an input instruction c. either a or b
b. an output instruction d. both a or b
20. The interconnection of timers is commonly
called :
a. grouping
b. programming
c. sequencing
d. cascading

70. 21. For the program shown, the pilot light will be:
on at all times
off at all times
c. switched on 15 s after the switch has been actuated
from the open to the closed position
d. switched off 15 s after the switch has been actuated
from the on to off position