Programmable
Logic Controllers
Third Edition
Frank D. Petruzella
McGraw-Hill
Chapter 7
Programming
Timers
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
On-Delay Timing Relay
Operating coil
NO
Instantaneous
contacts
NC
Nontimed contacts are
controlled directly by
the timer coil, as in a
general-purpose
control relay.
When the coil is
energized, the timed
contacts are are
prevented from opening
NO
or closing until the time
Time control delay period has elapsed.
However, when the coil is
contacts
de-energized, the timed
NC
contacts return
Time adjustment instantaneously to their
normal state.
Timed Contact Symbols
On-Delay Symbols
Normally open, timed
closed contact (NOTC)
Normally closed, timed
open contact (NCTO)
Contact is open when
relay coil is de-energized
Contact is closed when
relay coil is de-energized
When relay is energized,
there is a time delay in
closing
When relay is energized,
there is a time delay in
opening
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 deenergized, 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 deenergized, there is a time
delay before the contact
closes.
On-Delay Relay Timer Circuit (NOTC Contact)
L1
L2
S1
Sequence of operation
S1 open, TD de-energized,
TD1 open, L1 is off.
L1
S1 closes, TD energizes,
timing period starts,
TD1 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
S1 is opened, TD deenergizes, TD1 opens
instantly, L1 is switched off.
On-Delay Relay Timer Circuit (NCTO Contact)
L1
L2
S1
Sequence of operation
L1
S1 open, TD de-energized,
TD1 closed, L1 is on.
S1 closes, TD energizes,
timing period starts, TD1
is still closed, L1 is still on.
10 s
10 s
After 10 s, TD1 opens,
L1 is switched off.
Input
Output
On
Off
Timing Diagram
S1 is opened, TD de-energizes,
TD1 closes instantly, L1 is
switched on.
Off-Delay Relay Timer Circuit (NOTO Contact)
L1
L2
S1
Sequence of operation
S1 open, TD de-energized,
TD1 open, L1 is off.
L1
10 s
10 s
Input
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.
On
Output
Off
After 10 s, TD1 opens, L1 is
switched off.
Timing Diagram
Off-Delay Relay Timer Circuit (NCTC Contact)
L1
L2
Sequence of operation
S1
S1 open, TD de-energized,
TD1 closed, L1 is on.
L1
S1 closes, TD energizes,
TD1 opens instantly,
L1 is switched off.
10 s
10 s
S1 is opened,
TD de-energizes, timing
period starts, TD1 is still
open, L1 is still off.
Off
After 10 s, TD1 closes,
L1 is switched on.
Input
Output
On
Timing Diagram
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
(b) NO
(c) instantaneous
(d) NC
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?
6. Which of the following symbols represents a
normally closed timed open contact?
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
8. The timing diagram shown is that of an
10 s
Input
On
Output
Off
Timing Diagram
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)
9. In the circuit shown, the light will stay off
L1
L2
S1
L1
10 s
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
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
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
RSLogic Timer Commands
TON
Timer/Counter
Command
TON
Name
Timer On-Delay
Description
Counts time base
intervals when the
instruction is “true”
RSLogic Timer Commands
TOF
Timer/Counter
Command
TOF
Name
Timer Off-Delay
Description
Counts time base
intervals when the
instruction is “false”
RSLogic Timer Commands
RTO
RES
Timer/Counter
Command
Name
RTO
Retentive Timer
ON
RES
Reset
Description
Counts time base
intervals when the
instruction is “true” and
retains the accumulated
value when the
instruction goes "false" or
when power cycle occurs
When this instruction is
"true" it resets the count
of the RTO counter
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).
Coil-Formatted Timer Instruction
Contact determines
rung continuity
The timer assigned
an address
The type of timer
XXX
is specified
TON
Preset value PR:YYY
Time base
0.1 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.
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.
Retentive timer block
Preset time
Time base
Accumulated time
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
preset
When the
two become
equal
the output
Reset
linethe
resets
thetime.
the timer's
accumulated
value
to zero.
changes to logic 1.
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 Sequence
Rung condition
Input
True
False
Timed period
On delay
time duration
True
Timed output bit
Timer
False
OFF
ON
The timed output
becomes true
sometimes after the
timer rung becomes
true; hence the timer
is said to have an on
delay.
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
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
T4:0
EN
Enable bit true
EN
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.
TON
TIMER ON DELAY
Timer
T4:0
Preset
50
Accumulated
10
T4:0
EN
Timer-Timing bit true
TT
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
Timer
T4:0
Preset
50
Accumulated
10
50
T4:0
EN
DN
Done-bit changes state
DN
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
Allen-Bradley On-Delay Timer Instruction
The information to be entered includes:
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:0
1.0
15
0
EN
DN
Timer number which must come from the timer file.
Time base which is expressed in seconds.
Preset value which is the length of the time delay.
Accumulated value which is normally entered as 0.
On-Delay Timer Program
Ladder Logic Program
L2
L1
Input A
Input A
TON
TIMER ON DELAY
Timer
Time base
Preset
Accumulated
T4:0
1.0
10
0
10
EN
DN
Output B
G
Output C
R
Output D
Y
Output B
T4:0
EN
Output C
T4:0
TT
Output D
T4:0
DN
On-Delay Timer Program
Timing Diagram
On
Input condition A
Timer-enable bit
Timer-timing bit
Off
On
Off
On
Off
On
Timer-done bit
Off
Timer accumulated 0
value
4s
4s
On-Delay Timer Program
Timers are 3-word elements
Word
EN TT DN
Internal use
Preset value PRE
Accumulated value ACC
Word 0 is the control word
Word 1 stores the preset value
Word 2 stores the accumulated value
0
1
2
On-Delay Timer With Instantaneous Output
Relay Ladder Schematic Diagram
L1
Stop
Start
L2
1TD
1TD-1
(instantaneous
contact)
M
1TD-2
(5 s)
(timed contact)
On-Delay Timer With Instantaneous Output
Programmed Circuit
Ladder logic program
Inputs
Stop
Internal
relay
Start
Output
L2
L1
Stop
Motor
Start
Internal
relay
Motor
M
Timer
PR: 5
TB: 1 s
Output
line
M
Start-Up Warning Signal Circuit
Relay Ladder Schematic Diagram
L1
Start-up
PB1
Reset
PB2
CR1
CR1-1
1TD
CR1-2
Horn
CR1-3
1TD-1
(10 s)
L2
Start-Up Warning Signal Circuit
Programmed Circuit
Ladder logic program
Inputs
Output
Start-up
PB1
PB2
PB1
T4:0
Reset
PB2
EN
T4:0
TON
TIMER ON DELAY
Timer
T4:0
Time base
1.0
Preset
10
Accumulated
100
Horn
T4:0
DN
EN
EN
DN
Horn
Timed Closed Solenoid Value Program
Ladder logic program
Input
Output
L2
L1
Switch
SW_1
SW_1
timer_1.dn
TON
Timer On Delay
Timer
timer_1
Preset
12000
Accumulated 12000
0
EN
DN
Valve
Valve
Automatic Sequential Control System
L1
Relay Ladder
Schematic
Diagram
Start
PB2
Stop
PB1
L2
Lube oil
pump motor
OL
M1
M1-1
PS1
Main drive
motor
OL
M2
(lube oil
pressure switch)
1TD
OL
M3
1TD-1
(15 s)
Feed
motor
Automatic Sequential Control System
Programmed Circuit
Ladder logic program
Inputs
PB1
PB2
M1
Outputs
OL
PB1
M1
M1
PB2
PS1
OL
PS1
M2
M2
OL
M3
TON
TIMER ON DELAY
Timer
T4:0
Time base
1.0
Preset
15
15
Accumulated
0
T4:0
DN
EN
DN
M3
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.
Ladder logic program
Input
Output
L2
L1
S1
I:1.0/0
TOF
TIMER OFF DELAY
I:1.0/0
EN
TIMER
Time base
Preset
Accumulated
T4:3/DN
T4:3
1.0
15
0
15
DN
O:2.0/1
O:2.0/1
PL
Off-Delay Programmed Timer
Timing Diagram
True
Input condition
S1
False
Timed period
Off delay
timed duration
True (logic 1)
False (logic 0)
Timed output
O:2.0/1
Preset value = accumulated value
Off-Delay Timer Used To Switch Motors Off
Input
Ladder logic program
L1
5000
10000
15000
Output
L2
Pneumatic Off-Delay Timer
L1
Relay Ladder
Schematic
Diagram
L2
Programmed Pneumatic Off-Delay Timer
Equivalent Programmed Circuit
Input
Ladder logic program
Outputs
L1
L2
5
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.
Fluid Pumping Process Program
Ladder logic program
Inputs
Output
L1
L2
05
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.
Retentive On-Delay Timer Program
The PLC-programmed RETENTIVE ON-DELAY timer
(RTO) operates in the same way as the nonretentive ondelay timer (TON), with one major exception. This is a
retentive timer reset (RTR) instruction.
Same address
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.
Retentive On-Delay Timer Program
03
9
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.
Retentive On-delay Alarm Program
Ladder logic program
L2
L1
60000
0
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.
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.
Bearing Lubrication Program
10
0
15
10800
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.
Relay Schematic
Diagram
Three motors started
automatically in
sequence with a 20-s
time delay between
each motor startup.
Equivalent Time-Delayed Motor-Starting Program
20000
20000
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.
Annunciator Flasher Circuit
Cascading of Timers for Longer Time Delays
30000
12000
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
Green
25 s
Amber
5s
Control of Traffic Lights in One Direction
30
25
5
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
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)
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
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
17. The timer reset instruction must be addressed
to the same address as the ______ instruction.
a. TON
b. TOF
c. RTO
d. EN
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.
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
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