Programmable
Logic Controllers
Third Edition
Frank D. Petruzella
McGraw-Hill
Chapter 8
Programming
Counters
Counters
Common applications of counters include keeping track
of the number of items moving past a given point, and
determining the number of times a given action occurs.
A preset counter can control an
external circuit when its
counted total matches the
user-entered preset limits.
Mechanical Counters
Programmed counters can serve
the same functions as mechanical
counters.
Every time the actuating lever is moved
over the counter adds one number,
while the actuating lever returns
automatically to its original position.
Resetting to zero is done with a
pushbutton located on the side of the
unit.
Electronic Counters
Electronic counters can count up, count down, or be combined
to count up and down. They are dependent on external
sources, such as parts traveling past a sensor or actuating a
limit switch for counting.
Counter
Applications
Coil-Formatted Counter Instruction
The coil is assigned an
address as well as being
identified as a counter
Increments
counter by 1 for
every false-to-true
transition
Like the timer, preset and
accumulated values are
included
When the accumulated count equals the preset count, the
output is energized and the counter output is closed. The
counter contact can be used as many times as you wish
throughout the program as an NO or NC contact.
Coil-Formatted Counter And Reset Instruction
Same address
Resets counter when true
Block-Formatted Counter Instruction
Count line
Type of
counter
Output
line
Preset value
Reset line
Accumulated
value
PLC counters operate or count on the leading edge of the
input signal. The counter will either increment or decrement
whenever the count input transfers from an "off" state to an
"on" state. The counter will not operate on the trailing edge,
or on-to-off transition of the input condition.
Counter Counting Sequence
PLC counters are normally retentive. Whatever
count was contained in the counter at the time of a
processor shutdown will be restored to the counter on
power-up. The counter may be reset, however, if the
reset condition is activated at the time of power
restoration.
PLC counters can be designed to count up to a preset
value or to count down to a preset value.
Counter Counting Sequence
The up-counter is incremented by 1 each time the rung
containing the counter is energized.
The counter will increment until the accumulated value is
equal to or greater than the preset value, at which time an
output will be produced.
Counter Counting Sequence
The down-counter decrements by 1 each time the rung
containing the counter is energized.
A counter reset is always provided to cause the counter
accumulated value to be reset to a predetermined value.
Simple Up-counter Program
7
Up-counter Program Timing Diagram
C5 Counter Data File
Each counter address is made of a 3-word element
•Bit 0-9: Internal Use
•Bit 10:
UAthe
- Update
accumulation
value. the controller
Specifies the value,
which
counter
must reach before
This is the number
of
ofUnderflow
false to true
transitions that have occurred
•Bit
11:times
UN
bit.
sets the done bit.
When
the-accumulated
value becomes equal to or
since the counter
was
reset.
•Bit
12:last
OV
- Overflow
greater than the
preset
value,
the donebit.
status bit is set. You can use this
•Bit
13: DN
- Done
bit to control an
output
device.
•Bit 14: CD - Count down is enabled.
•Bit 15: CU - Count up is enabled.
PLC-5 And SLC 500 Count-Up Counter Instruction
ControlLogix Count-Up Counter Instruction
The counter address in the PLC-5 and SLC 500 is a data table
address, whereas in the ControlLogix it is a predefined
structure of the data type.
In the PLC-5 and SLC 500, the max value for the preset and
accumulated values is 32,767 and the min value is –32,768; for
the ControlLogix controller the max value is 2,147,438,647 and
the min value is –2,147,438,648.
RSLogic Counter Commands
Command
RES
HSC
CTD
CTU
Name
Description
Decrements
the
accumulated
Increments
the
accumulated
Resets
the
accumulated
Counts high-speed pulses
Count-Down
Count-Up
value
ataand
each
false-to-true
value
at
each
false-to-true
High-Speed
Reset
value
status
bit of
from
fixed
controller
transition
andinput
retains the
the
transition
and
retains
Counter
the counter
high-speed
accumulated value
value when
when
accumulated
power cycle
cycle occurs
occurs
power
Parts Counting Program
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 per day
A pushbutton is used to restart the total part and package count
from zero daily
Parts Counting Program
9
10
15
10
59
1
1. One common application for a counter is
keeping track of the number of items moving
past a given point. (True/False)
2. Timers, like counters, are not dependent on
external sources for counting. (True/False)
3. The down-counter increments by 1 each time
the rung containing the counter is energized.
(True/False)
4. Normally, when power to your PLC
system is lost, when power is restored the
counter accumulated value will be:
a. zero
b. whatever it was preset for
c. maximum
d. unchanged
5. The output of a PLC counter is switch from “off”
to “on” any time the count input rung is "true".
(True/False)
6. A PLC up-counter normally counts true-to-false
transitions. (True/False)
7. In order for the PLC counter to reset, the
counter reset rung must be “false”.
(True/False)
8. The done bit of a counter is true whenever:
a. the accumulated value is equal to the preset
value
b. the counter instruction is true
c. the accumulated value is greater than the
preset value.
d. both a and c
9. The _________ value of a PLC counter
is the current count based on the number
of times the rung goes from false-to-true.
a. preset
c. accumulated
b. overflow
d. underflow
10. Count rung transitions can be caused by
actuating:
a. a limit switch
c. a pressure switch
b. a sensor
d. any of these
One-Shot, Or Transitional, Contact Program
The transitional or one shot contact program can be used to
automatically clear or reset a counter. The program is designed
to generate an output pulse that, when triggered, goes on for
the duration of one program scan and then goes off.
One-Shot, Or Transitional, Contact Program
The transitional or one shot contact program can be used to
automatically clear or reset a counter. The program is designed
to generate an output pulse that, when triggered, goes on for
the duration of one program scan and then goes off.
The one-shot can be
triggered from a
momentary signal, or
one that comes on and
stays on for some time.
Types Of Transitional Contacts
Off-To-On Transitional Contact
On
Off
On
Symbol
One
scan
Off
Is programmed to
provide a one-shot
pulse when the
referenced trigger
signal makes a
positive (off-to-on)
transition
On-To-Off Transitional Contact
On
Off
On
Symbol
Off
One
scan
Is programmed to
provide a one-shot
pulse when the
referenced trigger
signal makes a negative
(on-to-off) transition
Conveyor Motor Circuit That Uses A
Programmed One-Shot Reset Circuit
Proximity
switch
Sequential Task:
The start button is pressed
to start the conveyor motor
Case
Conveyor motor
Start/Stop station
Count reset
button
Cases move pass the proximity
switch and increment the
counter's accumulated value
After a count of 50, the conveyor motor stops automatically
and the counter's accumulated value is reset to zero
The conveyor motor can be stopped or started manually at
anytime without loss of the accumulated count
Conveyor Motor Circuit That Uses A
Programmed One-Shot Reset Circuit
50
One-Shot Rising (OSR) Instruction
• Triggers a one-time event.
• The OSR instruction is a retentive input instruction
that triggers an event to occur only one time. Use the
OSR instruction when an event must start based on
change of state of the rung from false to true.
• When the input instruction goes from false to true, the
OSR instruction conditions the rung so that the output
goes true for one scan. The output goes false and
remains false for successive scans until the input makes
another false to true transition.
Alarm Monitor Program
 The alarm is triggered by the closing of liquid level switch
LS1
 The light will flash whenever the alarm condition is triggered
and has not been acknowledged, even if the alarm condition
clears in the meantime
 The alarm is acknowledged by closing selector switch SS1
 The light will operate in the steady mode when the alarm
trigger condition exists but has been acknowledged
Alarm Monitor Program
Down-Counter
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 downcounter is used in conjunction with the up counter to
form an up/down counter.
Generic up/down
counter program
Up/Down Counter Timing diagram
Preset Value = 3
Parking Garage Counter Program
 As a car enters, it triggers the up-counter output instruction and
increments the accumulated count by 1.
 As a car leaves, it triggers the down-counter output
instruction and decrements the accumulated count by 1.
 Since both the up- and down-counters have the same address,
the accumulated value will be the same in both.
 Whenever the accumulated value equals the preset value, the
counter output is energized to light up the Lot Full sign.
Parking Garage Counter Program
50
38
150
50
150
38
PLC-5 And SLC-500
Count-Down Counter Instruction
If the accumulation value is below the
minimum range then the underflow
(UN) bit will be true.
Up/Down-Counter Program
1
10
1
10
When
the
CTU
Whenthe
theaccumulated
CTD instruction
instruction
When
value
is
true,
C5:2/CU
will
be
true
isgreater
true,CC5:2/CD
will
beto
true
going
true
will
cause
isInput
than
or
equal
the
causing
output
A
to
be
true
causing
output
B
to
be
true
both counter
instructions
preset
value, C5:2/DN
will to
be
resetcausing output C to be
true,
true
In-Process Monitoring System
After
processing,
finished
parts
appearing empty
at the out-feed
sensor
Before
start-up,
the
system
is
completely
of
parts,
and
When
the
operation
begins,
raw
parts
move
through
the
in-feed
generate
down
counts,
so the accumulated
count of the counter
the
counter
is
reset
manually
to
zero.
sensor, with each
part generating
count. parts.
continuously
indicates
the numberan
ofup
in-process
In-Process Monitoring System
58
8
5
Counting Beyond The Maximum Count
15000
Counter Speed
The maximum speed of transitions you can count is
determined by your program's scan time. Any counter
input signal must be fixed for one scan time to be
counted reliably.
If the input changes faster
than one scan period, the
count value will become
unreliable because counts
will be missed. When this is
the case you need to use a
high-speed counter.
Cascading Counters
Depending on the
application, it may be
necessary to count events
that exceed the maximum
number allowable per
counter instruction. One way
of accomplishing this is by
interconnection, or
cascading, two counters.
Counting Beyond The Maximum Count
The status
bits of both
counters are
programmed
in series to
produce an
output
The output of the
first counter is
programmed into
the input of the
second counter
These two
counters allow
twice as many
counts to be
measured
Cascading Counters For Extremely Large Counts
500
0
1
Whenever counter C5:1
reaches 500, its done bit
resets counter C5:1 and
increments counter C5:2
by 1
The output light turns
on after 500 x 500, or
250,000 transitions of
the count input
24 Hour Clock Program
The timer times for a 60 s period,
after which its done bit is set. This,
in turn cases C5:0 to increment 1
count. On the next processor scan,
the timer is reset and begins timing
again.
Whenever C5:0 reaches its preset
value of 60, its done bit is set. This, in
turn causes it to reset itself and C5:1
to increment 1 count.
Whenever C5:1 reaches its preset
value of 24, its done bit is set to reset
itself.
Program For
Monitoring
The Time
Of An Event
Incremental Encoder
An incremental encoder creates a
series of square waves as its shaft
is rotated.
The encoder disk interrupts the light
as the encoder shaft is rotated to
produce the square wave output
waveform.
Incremental Encoder
The number of square waves obtained
from the output of the encoder can be
made to correspond to the mechanical
movement required.
To divide a shaft revolution into 100
parts, an encoder could be selected to
supply 100 square wave cycles per
revolution. By using a counter to
those cycles, we could tell how far
the shaft has rotated.
Cutting Objects To A Specific Size
The object is advanced for a specific distance and measured
by encoder pulses to determine the correct length for
cutting.
Counter Used For Length Measurement
Count input pulses are generated by the magnetic sensor, which
detects passing teeth on a conveyor drive sprocket. If 10 teeth per
foot of conveyor motion pass the sensor, the accumulated count of
the counter would indicate feet in tenths.
The photoelectric sensor monitors a reference point on the
conveyor. When activated, it prevents the unit from counting,
thus permitting the counter to accumulate counts only when bar
stock is moving.
Counter Used For Length Measurement
10
Photo sensor activated, therefore,
accumulated count remains at 10
Combining Counter And Timer Functions
When the start button is pressed,
conveyor M1 begins running.
After 15 plates have been stacked,
conveyor M1 stops and conveyor
M2 begins running.
After conveyor M2 has been
operated for 5 s, it stops and the
sequence is repeated
automatically.
Automatic Stacking Process
The done bit of the timer resets
the timer and counter, and
provides a momentary pulse to
automatically restart conveyor
M1.
Automatic
Stacking
Program
Motor Lock-Out Program
Designed to prevent
a machine operator
from starting a
motor that has
tripped off more
than 5 times in an
hour.
The normally open (OL) relay
contact momentarily closes
each time an overload current
is sensed.
Every time the the motor
stops due to an overload
condition, the motor start
circuit is locked out for 5 min.
If the motor trips off more
than 5 times in an hour, the
motor stat circuit is
permanently locked out and
cannot be started until the
reset button is actuated.
Motor
Lock-Out
Program
Product Flow Rate Program
Parts
This program is designed to indicate how
many parts per minute pass a given
process point.
Sensor
When the start switch is closed, both the counter and timer
are enabled.
The counter is pulsed for each part passing the sensor.
The counting begins and the timer starts timing through its
1-min time interval.
At the end of 1 min, the timer done bit causes the counter
rung to go false. Sensor pulses continue but do not affect the
PLC counter. The number of parts for the past minutes are
represented by the accumulated value of the counter.
Product Flow Rate Program
60
28
Timer Driving A Counter For Long Time-Delay Period
0
10,000
1
Each timer T4:0
input closes for
10,000 s, its done
bit resets itself
and increments
C5:0 by 1.
The output light turns on 10,000 x
100, or 1,000,000 seconds after the
timer input contact closes.
11. A _______ contact is designed to generate
an output pulse that, when triggered, goes on
for the duration of one program scan and then
goes off.
(a) examine if open
(b) examine if closed
(c) internal relay
(d) transitional
12. The down-counter output instruction will
increment by 1 each time the counted event
occurs. (True/False)
13. Normally the down-counter is used in
conjunction with the up counter. (True/False)
14. The individual up- and down-counters of
an up/down counter program must have the
same address.
(True/False)
15. PLC counters can be used for:
a. straight counting in a process
b. two counters used to give the difference
between two counts
c. two counters used to give the sum of the
two counts
d. all of these
16. Interconnecting or cascading two PLC counters
involves:
a. programming the outputs of each in parallel
b. hardwiring the two in series
c. programming the output of the first counter
into the input of the second counter
d. hardwiring the two in parallel
17. The incremental encoder creates:
a. a pure dc waveform
b. a sawtooth waveform
c. an ac sine waveform
d. a series of square waves
18. The maximum speed of transitions you
can count is determined by:
a. the amount of PLC memory
b. the preset value of the counter
c. your program's scan time
d. all of these
19. For the program shown, if the in-feed switch is
pulsed 9 times followed immediately by 5 out-feed
switch pulses, the accumulated count would be:
(a) 0
(b) 14
(c) 50
(d) 4
20. For the program shown, the light turns on ___
seconds after the timer input contact closes.
(a) 1,000,000
(b) 50,000,000
(c) 500,000
(d) 100,000