The document discusses various types of timers and counters used in programmable logic controllers (PLCs), including on-delay timers, off-delay timers, retentive timers, up counters, down counters, and accumulator registers. It provides examples of ladder logic diagrams using timers and counters to control sequences of motors and lights. Combining timers and counters allows automation of processes by timing delays between events and counting inputs.

1. On delay timer,
OFF delay timer,
retentive timer instruction,
PLC counter Up and Down instructions,
Combining counter and timers,
simple application program using timer and counters ,
FBD concepts and programming
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2.  Timer is an instruction that waits a set
amount of time before doing something
(control time).
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3.  PLC timers are output instructions that provide
the same functions as mechanical timing
relays.
 They are used to activate or deactivate a device
after a preset interval of time.
 The preset time represents the time duration
for the timing circuit.
 The accumulated time represents the amount
of time that has elapsed from the moment the
timing coil became energized.
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4.  The advantage of PLC timers is that their
settings can be altered easily, or the number of
them used in a circuit can be increased or
decreased through the use of programming
changes rather than wiring changes.
 Another advantage of the PLC timer is that its
timer accuracy and repeatability are extremely
high since it is based on solid-state
technology.
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5.  On-Delay timer- simply “delays turning on”.
It is called TON
 Off-Delay timer- simply “delays turning off”.
It is called TOF
 Retentive or Accumulating timer- holds or
retains the current elapsed time when the
sensor turns off in mid-stream. It is called
RTO
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6.  A timer consists of an internal clock, a count
value register, and an accumulator. It is used
for or some timing purpose.
Clock
Accumulator
contact
reset
output
Register
Contact
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7.  Timers are output instructions that you can
condition with input instructions such as
examine if closed and examine if open.
 They time intervals as determined by your
application program logic.
 The on-delay timer operates such that when
the rung containing the timer is true, the
timer time-out period commences.
 At the end of the timer time-out period, an
output is made active.
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10. Enable (EN) bit
The enable 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) .
Timer-timing (TI) bit
◦ The timer-timing bit is true whenever the accumulated
value of the timer is changing, which means the timer is
timing.
◦ When the timer is not timing, the accumulated value is not
changing. so the timer-timing bit is false.
Done (ON) bit
The done bit changes state whenever the
accumulated value reaches the preset value. its state
depends on the type of timer being used.
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11. Timer number
 This number must come from the timer file. In the example
shown. the timer number is T4:0. which represents timer file 4.
timer 0 in that 6le. There may be up to 1000 timers in each timer
file. Numbered from 0 through 999. The timer address must be
unique for this timer and may not be used for any other timer.
Time base
 The time base (which is always expressed in seconds) may be
either 1.0 s or 0.01 s. In the example shown, the time base is 1.0
s.
Preset value
 the preset value is 15. The timer preset value can range from 0
through 32,767.
Accumulated value
 the accumulated value is o. The timer's accumulated value
normally is entered as 0, although it is possible to enter a value
from 0 through 32,767
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12.  a series of motors can be started automatically with only
one start/stop control station.
 According to the relay ladder schematic, lube-oil pump
motor starter coil M1 is energized when the start
pushbutton PB2 is momentarily actuated.
 As a result,M1-1 control contact closes to seal in M1. and
the lube-oil pump motor starts.
 When the lube-oil pump builds up sufficient oil pressure.
the lube-oil pressure switch PS1 closes.
 This in turn energizes coil M2 to start the main drive
motor and energizes coil TD-1 to begin the time-delay
period.
 After the preset time-delay period of 15 s. 1 TD-l contact
closes to energize coil M3 and start the feed motor.
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15.  To write the ladder logic to control the
motors in section. The conditions are as
follows:
 Motor 1 should be turned ON after 5 seconds
the main switch has been switched ON.
 Motor 2 should be turned ON for 10 seconds
after Motor 1 s turned ON.
 Motor 3 should be turned ON after Motor 2 is
turned OFF.
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17.  The off-delay timer (TOF) operation will keep
the output energized for a time period after
the rung containing the timer has gone false.
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19.  If logic continuity is lost. the timer begins
counting time-based intervals until the
accumulated time equals the programmed preset
value.
 When the switch connected to input I:1.0/0 is
first closed, timed output 0 :2.0/1 is set to 1
immediately and the lamp is switched on.
 If this switch is now opened. logic continuity is
lost and the timer begins counting. After 15 s,
when the accumulated time equals the preset
time.
 the output is reset to 0 and the lamp switches
off. If logic continuity is gained before the timer
is timed out. The accumulated time is reset to O.
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20.  when power is first applied (limit switch LS1 open). Motor
starter coil M1 is energized and the green pilot light is on.
 At the same time, motor starter coil M2 is de-energized.
and the red pilot light is off.
 When limit switch LS1 closes. off-delay timer coil TD1
energizes.
 As a result, timed contact TD1-1 opens to de-energize
motor starter coil M1. timed contact TD1-2 closes to
energize motor starter coil M2.
 instantaneous contact T01-3 opens to switch the green
light off. and instantaneous contact TOI-4 closes to switch
the red light on.
 The circuit remains in this state as long as limit switch LSI
is closed.
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23.  When limit switch LSI is opened , the off delay
timer coil TD1 de-energizes.
 As a result, the time-delay period is started,
instantaneous contact TD1-3 closes to switch
the green light on, and instantaneous contact
TD1-4 opens to switch the red light off.
 After a 5-s time-delay period, timed contact
TD1-1 closes to energize motor starter MI.
and timed contact TD1-2 opens to de-
energize motor starter M2.
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24.  The process involves pumping fluid from tank A to
tank B. The operation of the process can be
described as follows:
 Before starting. PSI must be closed.
 When the start button is pushed. 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 5s after the pump
starts. If either PS2 or PS3 opens. the pump will shut
off and will not be able to start again for another 14
s.
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PS 1
STOP
PS2
PS3

26.  A retentive timer accumulates time whenever the
device receives power, and it 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. to the device after reaching its preset value
does not affect the state of the contacts.
 The retentive timer must be intentionally reset
with a separate signal for the accumulated time
to be reset and for the contacts of the device to
return to their self state.
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27.  one major exception-a retentive timer reset (RES)
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 instruction to reset the
accumulated value of the timer to O.
 The RES instruction is the only automatic means of
resetting the accumulated value of a retentive timer.
 The RES instruction has the same address as the
timer it is to reset. Whenever the RES instruction is
true.
 both the timer accumulated value and the timer done
bit (ON) are rest to O .
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28.  PLC program for a retentive on-delay time along with a timing
chart for the circuit.
 The timer will start to time when time pushbutton PB1 is closed.
 If the pushbutton is opened 3 s, the timer accumulated value
stays at 3 s. When the time pushbutton is closed again, the timer
picks up the time at 3 S and continues timing.
 When the accumulated value equals the preset value, the timer
done bit T4 :2/DN is set to 1 and the pilot light output PL is
switched on.
 Because the retentive timer does not reset to 0 when the timer is
de-energized. the reset instruction RES must be used to reset
the timer.
 The RES instruction given the same address (T4:2) as the RTO.
When reset pushbutton PB2 closes. RES resets the accumulated
time to 0 and the ON bit to O. turning pilot light PL off.
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31.  A counter is set to some preset value and,
when this value of input pulses has been
received, it will operate its contacts.
 The counter accumulated value ONLY
changes at the off to on transition of the
pulse input.
 Typically counters can count from 0 to 9999,
-32,768 to +32,767 or 0 to 65535.
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32.  Up-counters counts from zero up to the
preset value. These are called CTU, CNT, C, or
CTR.
 Down-counters count down from the preset
value to zero. These are called CTD.
 Up-down counters count up and/or down.
These are called CTUD.
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33.  The count-up counter is an output
instruction whose function is to increment its
accumulated value on false-to-true
transitions of its instruction.
 It can be used to count false -to-true
transitions of an input instruction and then
trigger an event after a required number of
counts or transitions.
 The up-counter output instruction will
increment by 1 each time the counted event
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36.  The countup enable bit is used with the
Count-up counter and is true whenever the
count-up counter instruction is true.
 If the Count-up counter instruction is false,
the CU bit is false.
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37.  The count-down enable bit is used with the
count-down counter and is true whenever the
count-down counter instruction is true.
 If the count-down counter instruction is false.
the CD bit is false.
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38.  The done bit is true whenever the
accumulated value is equal to or greater than
the preset value of the counter, for either the
count-up or the countdown counter.
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39.  The overflow bit is true whenever the counter
counts past its maximum value, which is
32,767.
 On the next count, the counter will wrap
around to -32,768 and will continue counting
from there toward 0 on successive false-to-
true transitions of the count up counter.
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40.  The underflow bit will go true when the
counter counts below -32,768. The counter
will wrap around to +32,767 and continue
counting down toward 0 on successive false-
to-true rung transitions of the count-down
counter.
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41.  The update accumulator bit is used only in
conjunction with an external HSC (high-speed
counter).
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42.  The preset value (PRE) word specifies the value that the counter
must count to before it changes the state of the done bit The
preset value is the set point of the counter and ranges Internal
Use (not addressable) from -32.768 through +32,767.
 The number is stored in binary form. with any negative numbers
being stored in 2's-complement binary.
 The accumulated value (ACC) lword is the current count based on
the number of times the rung goes from false to true.
 The accumulated value either increments with a false-to true
transition of the count-up counter instruction or decrements
with a false-to-true transition of the count-down counter
instruction.
 It has the same range as the preset: -32,768 through +32 ,767.
The accumulated
 value will continue to count past the preset value instead of
stopping at the preset like a timer does.
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43.  This control application is designed to turn the red pilot light on
and the green pilot light off after an accumulated count of 7.
 Operating pushbutton PB1 provides the off-to-on transition
pulses that are counted by the counter.
 The preset value of the counter is set for 7. Each false-to-true
transition of rung 1 increases the counter's accumulated value by
1.
 After 7 pulses, or counts. when the preset counter value equals
the accumulated counter value, output ON is energized.
 As a result, rung 2 becomes true and energizes output 0:2/0 to
switch the red pilot light on.
 At the same time, rung 3 becomes false and de-energizes output
0:2/ 1 to switch the green pilot light off.
 The counter is reset by closing pushbutton PB2, which makes
rung 4 true and resets the accumulated count to zero.
 Counting can resume when rung 4 goes false again.
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46.  PLC parts-counting program that uses three up-
counters.
 Counter C5:2 counts the total number of parts
coming off an assembly line for final packaging.
 Each package must contain 10 parts. When 10
parts are detected, counter C5:1 sets bit B3/1 to
initiate the box closing sequence.
 Counter C5:3 counts the total number of
packages filled in a day.
 (The maximum number of packages per day is
300.)
 A pushbutton is used to restart the total part and
package count from zero daily.
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48.  The down-counter output instruction will
count down or decrement by 1 each time the
counted event occurs.
 Each time the down count event occurs. the
accumulated value is decremented. Normally
the down-counter is used in conjunction with
the up-counter to form an up/down-counter.
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50.  Assuming the preset value of the counter is 3 and the
accumulated count is O. pulsing the count-up input (PB1 )
three times will switch the output light from off to on.
 This particular PLC counter keeps track of the number of
counts received above the preset value.
 As a result, three additional pulses of the count-up input
(PBt) produce an accumulated value of 6 but no change in
the output.
 If the count-down input (PB2) is now pulsed four times,
the accumulated count is reduced to 2 (6 - 4).
 As a result, The accumulated count drops below the preset
count and the output light switches from on to off.
 Pulsing the reset input (PB3) at any time will reset the
accumulated count to 0 and turn the output light off.
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52.  The CTD instruction decrements its accumulated
value by 1 every time it is transitioned.
 It sets its done bit when the accumulated value is
equal to or greater than the preset value.
 The CTD instruction requires the RES instruction
to reset its accumulated value and status bits.
 Because it resets its accumulated value to o.
 the CTD instruction then counts negative when it
transitions.
 If the CTD instruction were used by itself with a
positive preset value. its done bit would be
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54.  An up/down-counter program that will increase the
counter's accumulated value when pushbutton PB1 is
pressed and will decrease the counter's accumulated
value when pushbutton PB2 is pressed.
 Note that the same address is given to the up counter
instruction, the down-counter instruction, and the
reset instruction.
 All three instructions will be looking at the same
address in the counter file.
 When input A goes from false to true, one count is
added to the accumulated value.
 When input B goes from false to true, one count is
subtracted from the accumulated value.
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55.  The operation of the program can be summarized as
follows:
 C5:21DN will he true. causing output C to be true.
 When the CTU instruction is true, C5:2/CU will be
true, causing output A to be true.
 When the CTD instruction is true. C5:2 /CD will be
true. causing output B to be true.
 When the accumulated value is greater than or equal
to the preset value.
 Input C going true will cause both counter
instructions to reset. When reset by the RES
instruction. the accumulated value will be reset to 0
and the done bit will be reset.
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56. 6/14/2023 56
A
B
C
PB1
PB2
C

57.  Many PLC applications use both the counter
function and the timer function.
 an automatic stacking program that requires
both a timer and counter. In this process,
conveyor Ml is used to stack metal plates
onto conveyor M2.
 The photoelectric sensor provides an input
pulse to the PLC counter each time a metal
plate drops from conveyor Ml to M2.
 When 15 plates have been stacked. conveyor
M2 is activated for 5 s by the PLC timer.
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59. The operation of the program can be summarized
as follows:
 When the start button is pressed, conveyor Ml
begins running.
 After 15 plates have been stacked, conveyor Ml
stops and conveyor M2 begins running.
 After conveyor M2 has been operated for 5 s, it
stops and the sequence is repeated
automatically.
 The done hit of the timer resets the timer and
the counter and provides a momentary pulse to
automatically restart Conveyor M1.
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60. 6/14/2023 60
M2
M1
PB1
PE
PB2

61.  The Function Block Diagram (FBD) is a graphical
language for programmable logic controller
design, that can describe the function between
input variables and output variables.
 A function is described as a set of
elementary blocks.
 Input and output variables are connected
to blocks by connection lines.
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62.  A function block is a program instruction unit
which, when executed, yields one or more
output values. Thus, a block is represented in
the manner
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67.  A signal lamp is required to be switched on if a
pump is running and the pressure is satisfactory,
or if the lamp test switch is closed.
 For the inputs from the pump and the pressure
sensors we have an AND logic situation since
both are required if there is to be an output from
the lamp.
 We, however, have an OR logic situation with the
test switch in that it is required to give an output
of lamp on regardless of whether there is a signal
from the AND system.
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