PLC

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

2. Chapter 7
Programming
Timers

3. 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.
Timers
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. 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
Timed Contact Symbols
On-Delay Symbols
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. 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.
Timed Contact Symbols
Off Delay Symbols
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)
L1 L2
S1
L1
Sequence of operation
10 s
S1 open, TD de-energized,
TD1 open, L1 is off.
After 10 s, TD1 closes, L1 is
switched on.
S1 closes, TD energizes,
timing period starts,
TD1 still open, L1 is still off.
S1 is opened, TD de-
energizes, TD1 opens
instantly, L1 is switched off.
10 s
OFF
ON
Input
Output
Timing Diagram

8. On-Delay Relay Timer Circuit (NCTO Contact)
S1 open, TD de-energized,
TD1 closed, L1 is on.
Sequence of operation
L1 L2
L1
S1
10 s
After 10 s, TD1 opens,
L1 is switched off.
S1 closes, TD energizes,
timing period starts, TD1
is still closed, L1 is still on.
L1 L2
S1 is opened, TD de-energizes,
TD1 closes instantly, L1 is
switched on.
10 s
On
Off
Input
Output
Timing Diagram

9. Off-Delay Relay Timer Circuit (NOTO Contact)
L1 L2
S1
L1
Sequence of operation
S1 open, TD de-energized,
TD1 open, L1 is off.
S1 closes, TD energizes,
TD1 closes instantly,
L1 is switched on.
S1 is opened, TD de-energizes,
timing period starts, TD1 is
still closed, L1 is still on.
10 s
After 10 s, TD1 opens, L1 is
switched off.
10 s
Input
Output
Off
On
Timing Diagram

10. Off-Delay Relay Timer Circuit (NCTC Contact)
L1 L2
S1
L1
Sequence of operation
S1 open, TD de-energized,
TD1 closed, L1 is on.
S1 closes, TD energizes,
TD1 opens instantly,
L1 is switched off.
S1 is opened,
TD de-energizes, timing
period starts, TD1 is still
open, L1 is still off.
10 s
After 10 s, TD1 closes,
L1 is switched on.
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.
(a) timed (c) instantaneous
(b) 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
time delay in closing
d. when the relay coil is de-energized, there is a
time delay in opening

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
L1
10 s

17. 10. In the circuit shown, the light will stay on
a. as long as S1 is closed
b. 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
TON Timer On-Delay Counts time base
intervals when the
instruction is “true”

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

21. RSLogic Timer Commands
Command Name Description
RTO 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
RTO RES
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
The timer assigned
an address
XXX
TON
The type of timer
is specified
Preset value PR:YYY
Time base 0.1 s
Accumulated value AC:000
Contact determines
rung continuity
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.
Preset time
Time base
Accumulated time
Retentive timer block
Control line controls the actual timing operation of the timer.
Whenever this line is true the timer will time.
Reset line resets the the timer's accumulated value to zero.
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.

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
False
OFF
True
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
0 0 0

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
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 50
Accumulated 10
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.
TIMER ON DELAY
TON
Timer T4:0
Preset 50
Accumulated 10
EN
DN
T4:0
DN
Done-bit changes state
50

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
0 0 0 0 0

31. Allen-Bradley On-Delay Timer Instruction
TON
TIMER ON DELAY
EN
DN
The information to be entered includes:
Timer T4:0
Timer number which must come from the timer file.
Time base 1.0
Time base which is expressed in seconds.
Preset 15
Preset value which is the length of the time delay.
Accumulated 0
Accumulated value which is normally entered as 0.

32. On-Delay Timer Program
Ladder Logic Program
L1 L2
Input A
Input A
T4:0
T4:0
T4:0
EN
TT
DN
TON
TIMER ON DELAY
Timer T4:0
Time base 1.0
Preset 10
Accumulated 0
EN
DN
Output B
Output B
Output C
Output C
Output D
Output D
G
R
Y
10

33. On-Delay Timer Program
Timing Diagram
Input condition A
On
Off
Timer-enable bit
On
Off
Timer-timing bit
On
Off
Timer-done bit
On
Off
Timer accumulated
value
0
4 s 4 s

34. On-Delay Timer Program
Timers are 3-word elements
Word
0
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
Motor
M
Internal
relay
Internal
relay
Stop
Stop
Start
Start
Timer
PR: 5
TB: 1 s Output
line
M
L1 L2
Inputs Output
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. TON
TIMER ON DELAY
Timer T4:0
Time base 1.0
Preset 10
Accumulated 0
Inputs
Output
Ladder logic program
Programmed Circuit
Horn
Horn
Start-up
Reset
PB1
PB1
PB2
PB2
EN
DN
T4:0
EN
DN
T4:0
EN
T4:0
Start-Up Warning Signal Circuit
10

39. Timed Closed Solenoid Value Program
Input
L1
Switch
SW_1
SW_1
Ladder logic program
Output
L2
Valve
Valve
EN
DN
timer_1.dn
TON
Timer On Delay
Timer timer_1
Preset 12000
Accumulated 0
12000

40. Automatic Sequential Control System
Relay Ladder
Schematic
Diagram
L1 L2
Stop
PB1
Start
PB2 Lube oil
pump motor
OL
M1
M1-1
PS1
(lube oil
pressure switch)
Main drive
motor
M2
OL
1TD
1TD-1
(15 s)
Feed
motor
OL
M3

41. Automatic Sequential Control System
Programmed Circuit
Inputs Outputs
Ladder logic program
OL
OL
OL
M1
M1
M2
M2
M3
M3
TON
TIMER ON DELAY
Timer T4:0
Time base 1.0
Preset 15
Accumulated 0
PB1
PB1
PB2
PB2
PS1 PS1
EN
DN
M1
T4:0
DN
15

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 1.0
Preset 15
Accumulated 0
I:1.0/0
I:1.0/0
O:2.0/1
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. L1
Off-Delay Timer Used To Switch Motors Off
Input
L2
Output
Ladder logic program
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
 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.
Operation
 Before starting, PS1 must be closed.
 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
L1 L2
Inputs Output
Ladder logic program
5
0

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 Program
3
9
0

52. Retentive On-Delay Timing Chart
Enable bit is reset when
input pushbutton PB1 is
opened
Accum value retained
when rung goes false
Accum = Preset
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
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.
60000
0
L1
L2
Ladder logic program

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
10
0
15
10800

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
30000
12000

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 Direction
30
25
5

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:
a. on at all times
b. 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