Bryan L. Programmable controllers. Theory and implementation
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Figure 9-29. Ladder rung where the outputs turn ON in different scans.
therefore, output 103 stays OFF. (2) In the second rung, contact 101 is OFF;
therefore, output 102 remains OFF. (3) In the third rung, contact 100 is OFF,
so output 101 remains OFF. (4) In the fourth rung, contact 10 is ON because
the push button is pushed, so output 100 turns ON. In the next scan (second),
if the push button remains ON, output 101 will turn ON because, at the end
of the first scan, the reference address 100 was set to logic 1. This logic will
continue until the fourth scan, when all four outputs will be ON. The outputs
will turn OFF in the same way once the push button is released.
The physical operation of a circuit like the one in Figure 9-29 is almost
impossible to observe while a PLC is running the control program because a
PLC completes its scan in milliseconds. All the pilot lights would seem to
come ON at the same time, even if they actually came on in different scans.
The only way to observe the ladder outputs would be to use single-scan PLC
operation. With single-scan operation, the processor reads the inputs, ex-
ecutes the logic, updates the outputs, and stops until another single scan is
executed. Single-scan operation is generally used during the testing of a
control program.
The important thing to remember about a ladder program is that for an output
to have an effect on another rung in the same scan, it must be programmed
before that rung. If it is not, order of execution problems can arise, especially
when using transitional contacts and one-shot outputs to reset and unlatch
other rungs. Figure 9-30 illustrates this type of programming order problem,
where the output unlatch instruction will never occur. Once contact 10 closes,
latching output 100, only the closing of contact 11 will unlatch the output.
When contact 12 closes, it triggers the one-shot output 11 (or the transitional
contact 12) for one scan. However, at the end of the scan, the one-shot output
turns OFF, so it is not able to unlatch coil 100 in the next scan.
1Scan 234567
10
103
102
101
100
102 103
101 102
100 101
10 100
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10 100
L
11 100
U
12 11
OS
10 100
L
11 100
U
12 11
(a) (b)
Figure 9-30. (a) The one-shot output and (b) the transitional contact will never unlatch
coil 100.
PROGRAMMING NORMALLY CLOSED INPUTS
So far in our discussion, we have tried to avoid presenting input device
connections that are in the normally closed condition. The reason for this is
simple—we did not want to confuse you. Understanding how to program a
normally closed input device is a difficult concept to comprehend at first.
Once you learn it, try explaining it to someone else and watch their reaction.
To explain how to program normally closed inputs, let’s look at the
following example. Suppose we want to implement logic identical to the
simple hardwired circuit shown in Figure 9-31. Implementing the same logic
means that the pilot light PL1 in the PLC should behave in the same manner
as the one in the hardwired circuit—if PB1 is not pushed, PL1 will be ON; if
PB1 is pushed, PL1 will be OFF. Figures 9-32 and 9-33 show two possible
methods for programming PB1 and implementing the logic. At first glance,
you may think that the solution in Figure 9-32 is the answer, but that is not
true; Figure 9-33 is the correct implementation.
L1 L2
PL1
PB1
L1 L1 L2L2
10
10 100
PL1
PB1
100
Figure 9-31. Hardwired logic.
Figure 9-32. Logic implementation with PB1 programmed as a normally closed contact.
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L1 L1 L2L2
10
10 100
PL1
PB1
100
Figure 9-33. Logic implementation with PB1 programmed as a normally open contact.
In Figure 9-32, the reference address of PB1 (input 10) is programmed as a
normally closed contact (examine OFF) that drives output coil 100, which is
connected to pilot light PL1. When the PLC starts, it reads the status of the
input device connected to input 10 and stores this data in the input table. If
PB1 is not pushed (see Figure 9-34a), the processor reads input 10 as logic 1
(power flowing to the module). During the execution of the ladder logic, the
PLC will evaluate the examine-OFF instruction, and since the reference
(input 10) is ON, it will open the normally closed contact, disrupting
continuity. Thus, output 100 will be OFF, and PL1 will not turn ON.
Conversely, if PB1 is pushed (see Figure 9-34b), the input module at location
10 will be logic 0 (power not flowing to the module). The processor’s
examination for an OFF condition at reference 10 will then be TRUE;
therefore, the instruction will provide continuity to the rung and turn output
100 and PL1 ON.
L1 L1 L2L2
10
10 100
PL1100
PB1
ON
(a)
L1 L1 L2L2
10
10 100
PL1100
PB1
Pushed
OFF
(b)
Figure 9-34. Power flow through the circuit shown in Figure 9-32 with (a) PB1 not pushed
and (b) PB1 pushed.
In Figure 9-33, the normally closed input condition has been programmed as
an examine-ON instruction. During operation (see Figure 9-35a), if PB1 is not
pushed, the input module 10 will read an ON status. When the processor
evaluates the ladder rung, its examination for an ON condition at reference 10
will be TRUE. Therefore, contact 10 will close to provide power to the rung,
turning output 100 and PL1 ON. On the other hand, if PB1 is pushed (see
Figure 9-35b), the input will have an OFF status and the processor will store
a logic 0 in the input table. During the evaluation of the rung, the processor
will find its examination for an ON condition at reference 10 to be FALSE
(input 10 is OFF), and continuity will not occur because the contacts will
remain open. Thus, output 100 and PL1 will be OFF.
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The programming solution for a normally closed input connection, as shown
in Figure 9-33, exemplifies the following: for a normally closed wired input
device to behave as a normally closed device when connected, it must be
programmed as an examine-ON, or normally open, contact instruction.
Discrete inputs to a PLC can be made to act as normally open or normally
closed contacts, regardless of their original configuration. This ability to
examine a single device for either an open or closed state is the key to the
flexibility of PLCs—no matter how a device is wired (normally open or
normally closed), the controller can be programmed to perform the desired
action without changing the wiring. Remember that the programming state of
an input depends not only on how it is wired, but also on the desired control
action. The following example shows a case in which the PLC programming
of one push button with two contacts differs depending on which contact is
wired to the module.
L1 L1 L2L2
10
10 100
PL1100
PB1
ON
(a)
L1 L1 L2L2
10
10 100
PL1100
PB1
Pushed
OFF
(b)
Figure 9-35. Power flow through the circuit shown in Figure 9-33 with (a) PB1 not pushed
and (b) PB1 pushed.
EXAMPLE 9-4
Show the PLC implementation of the hardwired logic shown in Figure
9-36 for the following scenarios using only one push button connec-
tion: (a) with the normally open contact connected to the input module
and (b) with the normally closed contact connected to the input
module. Describe the operation of each implementation as well.
Use input address 10 for the push button and addresses 30 and 31
for pilot lights PL1 and PL2, respectively. Indicate the lights in the ON
condition (without PB1 being pushed) using a shaded PL indicator.
S
OLUTION
Examining the circuit in Figure 9-36 shows that, if PB1 is not pushed,
PL1 should be OFF. PL2 should be ON because the other contact of
PB1 (the normally closed one) provides power to PL2. We can wire any
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of PB1’s two connections (A or B) to the input module to satisfy the
required logic. Remember that we can make any contact act as we
desire in the PLC program (i.e., as a normally open or normally closed
contact).
(a) Figure 9-37 shows the solution for the normally open contact
connection. An examine-ON instruction drives PL1, and an examine-
OFF instruction drives PL2. When PB1 contact A is not pushed, PL1 is
OFF and PL2 is ON. The first rung implements a push button wired as
normally open to act as a normally open push button, while the second
rung implements a push button wired as normally open to act as a
normally closed push button.
Figure 9-37. Normally open implementation of Figure 9-36.
(b) Figure 9-38 shows the circuit solution for the normally closed
contact connection. In this solution, an examine-OFF instruction drives
PL1. During operation, PB1 contact B provides power to the module
if it is not pushed; therefore, the reference address (10) is logic 1. The
normally closed contact with address 10 will be open as long as PB1
is not depressed, keeping PL1 (output 30) OFF. In the second rung,
an examine-ON instruction drives the output for PL2 (31), which is
Figure 9-36. Hardwired logic for Example 9-4.
L1 L2
PL1
PB1
A
PL2
B
Hardwired Logic
L1 L1 L2L2
10 30
PL1
PB1
30
10
AA
10 31
PL2
31
BB
Not
wired to
PLC
input
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Figure 9-38. Normally closed implementation of Figure 9-36.
closed as long as PB1 is not pushed. The first rung implements a push
button wired as normally closed to act as a normally open push button,
while the second rung implements a push button wired as normally
closed to act as a normally closed push button.
As illustrated in the previous example, a normally open input can be
programmed in a PLC to behave like a normally closed device and vice
versa. However, for fail-safe reasons, normally closed input devices should
be wired to the input module as normally closed devices and then pro-
grammed as examine-ON instructions, so that they behave like normally
closed devices. A wired normally open device must not be programmed to act
as a normally closed device, especially if it is being used to interrupt
continuity when a device is pushed or closed.
Figure 9-39a shows an example of a normally closed stop push button used
to stop the power to a motor. During operation, when the start PB has been
pressed and sealed by the internal motor contact (100), the motor turns ON
(see Figure 9-39b). The normally closed stop PB interrupts the power
continuity to the motor output coil contact. The pressing of this stop push
button is the only way the motor can be stopped (see Figure 9-39c). However,
if the wire connection for the stop PB is accidentally cut, the motor circuit
will disengage (see Figure 9-39d).
This same logic operation can also be achieved using a normally open stop
PB instead of a normally closed one and implementing it as a normally
closed circuit in the PLC program (see Figure 9-40a). When the start button
is pushed, the motor turns ON (see Figure 9-40b); if the stop PB is pressed,
the motor turns OFF (see Figure 9-40c). However, there is no way to stop the
motor from running if the normally open stop PB wire is cut (see Figure 9-
40d). The programmed examine-OFF instruction corresponding to the stop
PB will never disrupt continuity in this situation. The only way to stop the
motor is to shut down power to the whole PLC system. This type of PLC
system configuration is dangerous and should be avoided at all times.
L1 L1 L2L2
10 30
PL1
PB1
30
AA
10 31
PL2
31
BB
10
Not
wired
to PLC
input
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Figure 9-39. Normally closed stop push button programmed as normally open.
Figure 9-40. Normally open stop push button programmed as normally closed.
10
10 11 100
Motor100
Start
11
Stop
100
M
The normally closed stop push button is programmed as normally open. Contact
100 is used as an interlock with the start push button after the start is pushed.
When the start push button is pressed, the motor turns ON.
(a)
10
10 11 100
Motor100
Start
11
Stop
100
M
After the start push button is pressed and released, the motor remains ON.
(b)
10
10 11 100
Motor100
Start
11
Stop
100
M
If the stop push button is pressed when the motor is ON, the motor will turn OFF.
(c)
10
10 11 100
Motor100
Start
11
Stop
100
M
If the stop push button connection breaks when the motor is ON, the motor will
turn OFF.
(d)
10
10 11 100
Motor100
Start
11
Stop
100
M
10
10 11 100
Motor100
Start
11
Stop
100
M
10
10 11 100
Motor100
Start
11
Stop
100
M
10
10 11 100
Motor100
Start
11
Stop
100
M
The normally open stop push button is programmed as normally closed. When
the start push button is pressed, the motor turns ON.
(a)
After the start push button is pressed and released, the motor remains ON.
(b)
If the stop push button is pressed when the motor is ON, the motor will turn OFF.
(c)
If the stop push button connection breaks when the motor is ON, pressing the
stop push button will not turn the motor OFF. This is a dangerous situation.
(d)
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9-6 TIMERS AND COUNTERS
PLC timers and counters are internal instructions that provide the same
functions as hardware timers and counters. They activate or deactivate a
device after a time interval has expired or a count has reached a preset value.
Timer and counter instructions are generally considered internal outputs.
Like relay-type instructions, timer and counter instructions are fundamental
to the ladder diagram instruction set.
Timer instructions may have one or more time bases (TB) which they use
to time an event. The time base is the resolution, or accuracy, of the timer. For
instance, if a timer must time a 10 second event, the user must choose the
number of times the time base must be counted to get to 10 seconds.
Therefore, if the timer has a time base of 1 second, then the timer must count
ten times before it activates its output. This number of counts is referred to as
ticks. The most common time bases are 0.01 sec, 0.1 sec, and 1 sec. Table 9-
3 shows the number of ticks required for a 10 second count, based on
different time bases.
Table 9-3. Time bases.
Timers are used in applications to add a specific amount of delay to an output
in the program. Applications of PLC timers are innumerable, since they have
completely replaced hardware timers in automated control systems. As an
example, timers may be used to introduce a 0.01 second delay in a control
program. The program may require such a delay because the PLC turns ON
its outputs very quickly as compared to the hardwired relay system it is
replacing. This small delay will slow down the response of other components
so that proper operation occurs.
Counter instructions are used to count events, such as parts passing on a
conveyor, the number of times a solenoid is turned ON, etc. Counters, along
with timers, must have two values, a preset value and an accumulated value.
These values are stored in register or word locations in the data table. The
preset value is the target number of ticks or counting numbers that must be
achieved before the timer or counter turns its output ON. The accumulated
value is the current number of ticks (timer) or counts (counter) that have
elapsed during the timer or counter operation. The preset value is stored in a
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preset register, while the accumulated value is kept in an accumulated
register. Both of these registers are defined during the programming of the
instruction. Either the basic ladder format or the block instruction format
can be used to implement timers and counters.
EXAMPLE 9-5
During a machine modernization project, it is found that part of a relay
ladder circuit (see Figure 9-41), when translated into a PLC circuit,
does not work correctly. This malfunction is due to the fact that in the
hardwired circuit, relay CR5, which is driven by device LS4, had
enough delay time to synchronize with the rest of the circuit so that the
solenoid actuation was correct. Now that it has been implemented in
the PLC, CR5 no longer has this delay. The delay needed is estimated
at 3 AC cycles (60 Hz) and the time bases available in the PLC are 0.01,
0.1, and 1 sec. Which time base should be used to create the delay
and how many ticks must the delay last?
Figure 9-41. Example relay ladder circuit.
SOLUTION
The estimated delay of 3 AC cycles translates into 60 Hz (i.e., 60
cycles/sec). So:
1 cycle =
1
60
= 16.66 msec
3 cycles =
3
60
= 50.00 msec
Thus, the required delay is 50 msec. Therefore, the only time base
small enough to use is 0.01 sec. Using this time base, the timer must
count 5 ticks to create the delay.
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.
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Table 9-4. Timer instructions.
RTR
TON
TON
TOF
TOF
RTO
9-7 TIMER INSTRUCTIONS
Figure 9-42. (a) Block format and (b) ladder format timer instructions.
PLCs provide several types of timer instructions. However, PLC manufac-
turers may provide different definitions for each type of timer function
offered. Table 9-4 presents a list of typical timer instructions.
The function of the various timer instructions is essentially the same, differing
only in the type of output provided. Figure 9-42 illustrates the two formats
used for timers. A block format timer has one or two inputs, depending on the
programmable controller. These inputs are called the control line and the
Preset
Register
Time Base
Accumulated
Register
TMR
Control
Enable/Reset
Output 1
Output 2
TMR
Preset Reg
Accumulated Reg
Time Base
(a) (b)
Control