Bryan L. Programmable controllers. Theory and implementation
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enable/reset line. If the control line is TRUE (i.e., it has continuity) and the
enable line is also TRUE, the block function will start timing. A ladder format
timer generally has only one input, which is the control line. If the control line
is ON, the timer will start timing.
Common to both timer formats is the use of a preset register to hold the
preset value and an accumulated register to store the accumulated value.
Some PLCs allow the user to enter a constant value directly into the timer to
set the preset value. This particular value, however, must be entered into a
predefined register for that specific timer address.
A timer’s time base is selectable depending on the PLC used (e.g., 0.01 sec,
0.1 sec, 1.0 sec, etc.). When the accumulated tick count equals the preset
count, the timer executes its timing function and sets the output condition,
which depends on the type of timer used (e.g., ON-delay energize, etc.).
It is important to note that when PLC timers replace hardwired timers, they
replace the time-delay contacts associated with the timers, but not the
instantaneous contacts that may be available from a hardwired timer. Figure
9-43 illustrates an example showing both time-delay and instantaneous
Figure 9-43. Hardwired circuit with time-delay and instantaneous contacts.
L1 L2
TMR1
PB1
PS1
TMR1-1
TS1
FS1
CR1 LS1
SOL2
SOL1
CR1
CR2
TMR2
CR1
CR3
CR3
SOL3
TMR2-1
TMR1-2
PS2
3 sec
2 sec
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hardwired timer contacts. Timer TMR1 in line 1 has an instantaneous
contact in line 2 (TMR1-1), which is used to seal PB1, and a time-delay
contact (TMR1-2) in line 5. For this type of ladder logic translation into a
PLC program, the user must “trap” the timer through interlocking, so that
the instantaneous timer seal can be accomplished. Chapter 11 presents this
type of programming example.
Figure 9-44. ON-delay energize timer instruction.
ON-DELAY ENERGIZE TIMER
An ON-delay energize timer (TON) output instruction either provides time-
delayed action or measures the duration for which some event occurs. Once
the rung has continuity, the timer begins counting time-based intervals (ticks)
and counts down until the accumulated time equals the preset time. When
these two values are equal, the timer energizes the output and closes the timed-
out contact associated with the output (see Figure 9-44). The timed contact
can be used throughout the program as either a normally open or normally
closed contact. If logic continuity is lost before the timer times out, the timer
resets the accumulated register to zero.
ON-DELAY DE-ENERGIZE TIMER
An ON-delay de-energize timer (TON) instruction operates in a manner
similar to an ON-delay energize timer instruction, except that the timer’s
output is already ON. This instruction de-energizes the output once the rung
has continuity and the time interval has elapsed (accumulated register value
= preset register value). PLC manufacturers provide either ON-delay ener-
gize or ON-delay de-energize timers, since it is easy to program one from the
other. Figure 9-45 illustrates a timing diagram for both types of ON-delay
timer instructions.
10 TON 100
100 10
Reg 1000 = 50
Reg 1001 = xx
0.1 sec
Preset Register:
Accumulated Register:
Time Base:
Timer output 100 is energized
5 seconds after contact 10 closes.
TON
TON
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OFF-DELAY ENERGIZE TIMER
Figure 9-45. Timing diagram for (a) an ON-delay energize timer and (b) an ON-delay
de-energize timer.
Figure 9-46. OFF-delay energize timer instruction.
An OFF-delay energize timer (TOF) output instruction provides time-
delayed action. If the control line rung does not have continuity, the timer
begins counting time-based intervals until the accumulated time value equals the
programmed preset value. When these values are equal, the timer energizes
the output and closes the timed-out contact associated with the output (see
Figure 9-46). The timed contact can be used throughout the program as either
a normally open or normally closed contact. If logic continuity occurs before
the timer times out, the accumulated value resets to zero.
OFF-DELAY DE-ENERGIZE TIMER
An OFF-delay de-energize timer (TOF) instruction is similar to its OFF-delay
energize counterpart; however, this timer’s output is ON and will be de-
energized once the rung loses continuity and the time interval has elapsed
(accumulated register value = preset register value). Like ON-delay timers,
PLC manufacturers usually provide either OFF-delay energize or de-energize
timers. Figure 9-47 shows timing diagrams for both types of OFF-delay timers.
0Timer’s Control Input
1
0ON-Delay Energize
1
0ON-Delay De-energize
1
(a)
(b)
Delay
10 TOF 100
100 10
Reg 1000 = 50
Reg 1001 = xx
0.1 sec
Preset Register:
Accumulated Register:
Time Base:
Timer output 100 is turned ON
5 seconds after contact 10 opens.
TOF
TOF
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9-8 COUNTER INSTRUCTIONS
There are two basic types of counters: those that can count up and those that
can count down. Depending on the controller, the format of these counters
may vary. Some PLCs use the ladder format (output coil), while others use
functional block format. Figure 9-48 illustrates these two formats, while
Table 9-5 presents common counter instructions.
RETENTIVE ON-DELAY TIMER
A retentive ON-delay timer (RTO) output instruction is used if the timer’s
accumulated value must be retained even if logic continuity or system power
is lost. If any rung path has logic continuity, the timer begins counting time-
based intervals until the accumulated time equals the preset value. The
accumulated register retains this accumulated value, even if power or logic
continuity is lost before the timer has timed out. When the accumulated time
equals the preset time, the timer energizes the output and turns ON (closes)
the timed-out contact associated with the output. Again, these timer contacts
can be used throughout the program as normally open or normally closed
contacts. A retentive timer reset instruction resets a retentive timer’s
accumulated value.
A retentive timer reset (RTR) output instruction is the only way to automati-
cally reset the accumulated value of a retentive timer. If any rung path has
logic continuity, then this instruction resets the accumulated value of its
referenced retentive timer to zero. Note that the retentive timer reset address
will be the same as the retentive timer output instruction it is resetting.
RETENTIVE TIMER RESET
Figure 9-47. Timing diagram for (a) an OFF-delay energize timer and (b) an OFF-delay
de-energize timer.
0Timer’s Control Input
1
0OFF-Delay Energize
1
0OFF-Delay De-energize
1
(a)
(b)
Delay
RTO
RTR
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Figure 9-48. (a) Block format and (b) ladder format counter instructions.
Table 9-5. Counter instructions.
CTU
CTD
CTR
UP COUNTER
An up counter (CTU) output instruction adds a count, in increments of one,
every time its referenced event occurs. In a control application, this counter
turns a device ON or OFF after reaching a certain count (i.e., the preset value
Preset
Register
Accumulated
Register
CTR
Up
Reset
Down
Output 1
Output 2
Count = Preset
Count > Preset
(a)
CTU
Preset Reg
Accumulated Reg
Up Counter
CTD
Preset Reg
Accumulated Reg
Down Counter
CTR
Reset Counter
(b)
CTU
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in the preset register). Also, this counter can keep track of the number of parts
(e.g., filled bottles, machined parts, etc.) that pass a certain point. An up
counter increases its accumulated value (the count value in its accumulated
register) each time the up-count event makes an OFF-to-ON transition. When
the accumulated value reaches the preset value, the counter turns ON the output,
finishes the count, and closes the contact associated with the referenced
output. After the counter reaches the preset value, it either resets its accumu-
lated register to zero or continues its count for each OFF-to-ON transition,
depending on the controller. In the latter case, a reset instruction is used to
clear the accumulated value.
DOWN COUNTER
COUNTER RESET
A counter reset (CTR) output instruction resets up counter and down counter
accumulated values to zero. When programmed, a counter reset coil has the
same reference address as the corresponding up/down counter coils. If the
counter reset rung condition is TRUE, the reset instruction will clear the
referenced address. The reset line in a block format counter instruction sets
the accumulated count to zero (accumulated register = 0). Figure 9-49
illustrates a typical block-formatted counter rung with up, down, and reset
counter instructions. The counter will count up when contact 10 closes, count
down when contact 11 closes, and reset register 1003 to 0 when contact 12
closes. If the count is equal to 15 as a result of either an up or down count,
output 100 will be ON. If the contents of register 1003 are greater than 15,
output 101 will be ON. Output 102 will be ON if the accumulated count value
is less than 15.
A down counter (CTD) output instruction decreases the count value in its
accumulated register by one every time a certain event occurs. In practical
use, a down counter is used in conjunction with an up counter to form an up/
down counter, given that both counters have the same reference registers.
In an up/down counter, the down counter provides a way to correct data that
is input by the up counter. For example, while an up counter counts the
number of filled bottles that pass a certain point, a down counter with the
same reference address can subtract one from the accumulated count value
every time it senses an empty or improperly filled bottle. Depending on the
programmable controller, the down counter will either stop counting down
at zero or at a specified maximum negative value. In a block format
instruction, a down count occurs every time the down input of the counter
transitions from OFF to ON.
CTD
CTR
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Figure 9-49. Counter function block with up, down, and reset counter instructions.
EXAMPLE 9-6
Figure 9-50 illustrates a block counter instruction being used to count
parts as detected by a photoelectric eye (PE) input. The preset value
of counts is 500. Modify this circuit so that it will automatically reset
every time the counter reaches 500. Also, add the instructions neces-
sary to implement an output coil that indicates that the count has
reached 500.
SOLUTION
Figure 9-51 illustrates a circuit that will automatically reset the
counter. When the preset and accumulated counts are equal, the
counter output 100 turns ON, latching output 101 to indicate a
reached count. This same counter output resets the counter. Remem-
ber that the PLC has already evaluated all inputs, so the counter is
reset in the following scan. The previous input 11 is used to manually
unlatch output 101.
Figure 9-50. Functional block counter instruction.
Output 1
Reset
Down
Up
10
11
12
100 101
100
Output 2
101
102
Count = Preset
Count > Preset
Count < Preset
CTR
PR: 1002 = 15
AR: 1003 = xx
L1 L1 L2L2
10
10
11
100
PL
(Count =
Preset)
PE
11
Reset
CTR
Up
Down
Reset
=
>
<
PR = 500
AR = xxx
100
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Figure 9-52. Solution to Example 9-7.
Figure 9-51. Automatically resetting counter.
EXAMPLE 9-7
Referencing the solution to Example 9-6 (i.e., see Figure 9-51),
implement the count detection circuit using interlocking standard
outputs and contacts instead of latch/unlatch coils.
S
OLUTION
Figure 9-52 illustrates an interlocking circuit that latches, or traps,
the counter’s output, indicating that the count value has been reached.
Note that the reset push button (input 11) is programmed normally
closed from a normally open input device. If this input was of safety
importance, then the circuit would have incorporated a normally closed
push button (wired as normally closed) that was programmed as an
examine-OFF instruction.
L1 L1 L2L2
10
10
100
100
PL
(Count =
Preset)
100 101
PE
11
Reset
CTR
Up
Down
Reset
=
>
<
101
L
11 101
U
L1 L1 L2L2
10
10
100
100
PL
(Count =
Preset)
100 11 101
PE
11
Reset
CTR
Up
Down
Reset
=
>
<
101
101
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9-9 PROGRAM/FLOW CONTROL INSTRUCTIONS
Program/flow control instructions direct the flow of operations, as well as
the execution of instructions, within a ladder program. They perform these
functions using branching and return instructions, which are executed when
certain already programmed control logic conditions occur. Typically, pro-
gram/flow control instructions form a “fence” within a program. This fence
contains groups of other ladder instructions that are used to implement the
desired function. Figure 9-53 illustrates a fence created using program/flow
control instructions.
Figure 9-53. A fence created using a program/flow control instruction.
Some programmable controllers, depending on their capabilities and the
scope of their application, use several types of program/flow control instruc-
tions. These instructions allow the controller to efficiently perform special
user-programmed routines that are executed only when required. This re-
duces the scan time, thereby optimizing total system response.
Table 9-6 shows some of the most commonly used program/flow control
instructions. These instructions are generally used in pairs. When paired, the
first instruction starts the flow control change, sending the PLC to a special
routine of instructions in another section of the control program. The other
instruction returns the PLC to the program it was running when the flow
control change occurred.
Main Control
Program
Main Control
Program
Flow Control
Instruction
Fenced
Program
End Flow
Control
If the rung is TRUE, the fenced
program (routine) is executed.
If the rung is FALSE, the fenced
program is bypassed.
Fenced
Instructions
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Figure 9-54. Example of an MCR instruction.
MASTER CONTROL RELAY
A master control relay (MCR) output instruction activates or deactivates the
execution of a group or zone of ladder rungs (see Figure 9-54). An MCR rung
is used in conjunction with an END rung (discussed later) to fence a group of
Table 9-6. Program/flow control instructions.
MCR
ZCL
LBL
JMP
GOSUB
RET
Main Control
Program
Main Control
Program
Auto MCR 1
END 1
Fenced
MCR
Zone
If the AUTO input closes,
MCR 1 is energized and
the rungs inside the zone
are executed. If AUTO is
OFF, program execution
resumes at the first rung
after the END instruction.
MCR
ZCL
END