Ladder logic tutorial

LADDER LOGIC

"Ladder" diagrams
Ladder diagrams are specialized schematics commonly used to document industrial
control logic systems. They are called "ladder" diagrams because they resemble a ladder,
with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are
control circuits to represent. If we wanted to draw a simple ladder diagram showing a
lamp that is controlled by a hand switch, it would look like this:

The "L1" and "L2" designations refer to the two poles of a 120 VAC supply, unless
otherwise noted. L1 is the "hot" conductor, and L2 is the grounded ("neutral") conductor.
These designations have nothing to do with inductors, just to make things confusing. The
actual transformer or generator supplying power to this circuit is omitted for simplicity.
In reality, the circuit looks something like this:

Typically in industrial relay logic circuits, but not always, the operating voltage for the
switch contacts and relay coils will be 120 volts AC. Lower voltage AC and even DC
systems are sometimes built and documented according to "ladder" diagrams:

So long as the switch contacts and relay coils are all adequately rated, it really doesn't
matter what level of voltage is chosen for the system to operate with.

Note the number "1" on the wire between the switch and the lamp. In the real world, that
wire would be labeled with that number, using heat-shrink or adhesive tags, wherever it
was convenient to identify. Wires leading to the switch would be labeled "L1" and "1,"
respectively. Wires leading to the lamp would be labeled "1" and "L2," respectively.
These wire numbers make assembly and maintenance very easy. Each conductor has its
own unique wire number for the control system that it's used in. Wire numbers do not
change at any junction or node, even if wire size, color, or length changes going into or
out of a connection point. Of course, it is preferable to maintain consistent wire colors,
but this is not always practical. What matters is that any one, electrically continuous point
in a control circuit possesses the same wire number. Take this circuit section, for
example, with wire #25 as a single, electrically continuous point threading to many
different devices:

In ladder diagrams, the load device (lamp, relay coil, solenoid coil, etc.) is almost always
drawn at the right-hand side of the rung. While it doesn't matter electrically where the
relay coil is located within the rung, it does matter which end of the ladder's power
supply is grounded, for reliable operation.

Take for instance this circuit:

Here, the lamp (load) is located on the right-hand side of the rung, and so is the ground
connection for the power source. This is no accident or coincidence; rather, it is a
purposeful element of good design practice. Suppose that wire #1 were to accidently
come in contact with ground, the insulation of that wire having been rubbed off so that
the bare conductor came in contact with grounded, metal conduit. Our circuit would now
function like this:

With both sides of the lamp connected to ground, the lamp will be "shorted out" and
unable to receive power to light up. If the switch were to close, there would be a short-
circuit, immediately blowing the fuse.

However, consider what would happen to the circuit with the same fault (wire #1 coming
in contact with ground), except this time we'll swap the positions of switch and fuse (L2 is
still grounded):

This time the accidental grounding of wire #1 will force power to the lamp while the
switch will have no effect. It is much safer to have a system that blows a fuse in the event
of a ground fault than to have a system that uncontrollably energizes lamps, relays, or
solenoids in the event of the same fault. For this reason, the load(s) must always be
located nearest the grounded power conductor in the ladder diagram.

• REVIEW:
• Ladder diagrams (sometimes called "ladder logic") are a type of electrical
notation and symbology frequently used to illustrate how electromechanical
switches and relays are interconnected.
• The two vertical lines are called "rails" and attach to opposite poles of a power
supply, usually 120 volts AC. L1 designates the "hot" AC wire and L2 the
"neutral" (grounded) conductor.
• Horizontal lines in a ladder diagram are called "rungs," each one representing a
unique parallel circuit branch between the poles of the power supply.
• Typically, wires in control systems are marked with numbers and/or letters for
identification. The rule is, all permanently connected (electrically common) points
must bear the same label.

Digital logic functions
We can construct simply logic functions for our hypothetical lamp circuit, using multiple
contacts, and document these circuits quite easily and understandably with additional
rungs to our original "ladder." If we use standard binary notation for the status of the
switches and lamp (0 for un-actuated or de-energized; 1 for actuated or energized), a truth
table can be made to show how the logic works:

Now, the lamp will come on if either contact A or contact B is actuated, because all it
takes for the lamp to be energized is to have at least one path for current from wire L1 to
wire 1. What we have is a simple OR logic function, implemented with nothing more
than contacts and a lamp.

We can mimic the AND logic function by wiring the two contacts in series instead of
parallel:

Now, the lamp energizes only if contact A and contact B are simultaneously actuated. A
path exists for current from wire L1 to the lamp (wire 2) if and only if both switch
contacts are closed.

The logical inversion, or NOT, function can be performed on a contact input simply by
using a normally-closed contact instead of a normally-open contact:

Now, the lamp energizes if the contact is not actuated, and de-energizes when the contact
is actuated.

If we take our OR function and invert each "input" through the use of normally-closed
contacts, we will end up with a NAND function. In a special branch of mathematics
known as Boolean algebra, this effect of gate function identity changing with the
inversion of input signals is described by DeMorgan's Theorem, a subject to be explored
in more detail in a later chapter.

The lamp will be energized if either contact is un-actuated. It will go out only if both
contacts are actuated simultaneously.

Likewise, if we take our AND function and invert each "input" through the use of
normally-closed contacts, we will end up with a NOR function:

A pattern quickly reveals itself when ladder circuits are compared with their logic gate
counterparts:

• Parallel contacts are equivalent to an OR gate.
• Series contacts are equivalent to an AND gate.
• Normally-closed contacts are equivalent to a NOT gate (inverter).

We can build combinational logic functions by grouping contacts in series-parallel
arrangements, as well. In the following example, we have an Exclusive-OR function built
from a combination of AND, OR, and inverter (NOT) gates:

The top rung (NC contact A in series with NO contact B) is the equivalent of the top
NOT/AND gate combination. The bottom rung (NO contact A in series with NC contact
B) is the equivalent of the bottom NOT/AND gate combination. The parallel connection
between the two rungs at wire number 2 forms the equivalent of the OR gate, in allowing
either rung 1 or rung 2 to energize the lamp.

To make the Exclusive-OR function, we had to use two contacts per input: one for direct
input and the other for "inverted" input. The two "A" contacts are physically actuated by
the same mechanism, as are the two "B" contacts. The common association between
contacts is denoted by the label of the contact. There is no limit to how many contacts per
switch can be represented in a ladder diagram, as each new contact on any switch or relay
(either normally-open or normally-closed) used in the diagram is simply marked with the
same label.

Sometimes, multiple contacts on a single switch (or relay) are designated by a compound
labels, such as "A-1" and "A-2" instead of two "A" labels. This may be especially useful
if you want to specifically designate which set of contacts on each switch or relay is
being used for which part of a circuit. For simplicity's sake, I'll refrain from such
elaborate labeling in this lesson. If you see a common label for multiple contacts, you
know those contacts are all actuated by the same mechanism.

If we wish to invert the output of any switch-generated logic function, we must use a
relay with a normally-closed contact. For instance, if we want to energize a load based on
the inverse, or NOT, of a normally-open contact, we could do this:

We will call the relay, "control relay 1," or CR1. When the coil of CR1 (symbolized with
the pair of parentheses on the first rung) is energized, the contact on the second rung
opens, thus de-energizing the lamp. From switch A to the coil of CR1, the logic function
is non-inverted. The normally-closed contact actuated by relay coil CR1 provides a
logical inverter function to drive the lamp opposite that of the switch's actuation status.

Applying this inversion strategy to one of our inverted-input functions created earlier,
such as the OR-to-NAND, we can invert the output with a relay to create a non-inverted
function:

From the switches to the coil of CR1, the logical function is that of a NAND gate. CR1's
normally-closed contact provides one final inversion to turn the NAND function into an
AND function.

• REVIEW:
• Parallel contacts are logically equivalent to an OR gate.
• Series contacts are logically equivalent to an AND gate.
• Normally closed (N.C.) contacts are logically equivalent to a NOT gate.
• A relay must be used to invert the output of a logic gate function, while simple
normally-closed switch contacts are sufficient to represent inverted gate inputs.

Permissive and interlock circuits
A practical application of switch and relay logic is in control systems where several
process conditions have to be met before a piece of equipment is allowed to start. A good
example of this is burner control for large combustion furnaces. In order for the burners

in a large furnace to be started safely, the control system requests "permission" from
several process switches, including high and low fuel pressure, air fan flow check,
exhaust stack damper position, access door position, etc. Each process condition is called
a permissive, and each permissive switch contact is wired in series, so that if any one of
them detects an unsafe condition, the circuit will be opened:

If all permissive conditions are met, CR1 will energize and the green lamp will be lit. In
real life, more than just a green lamp would be energized: usually a control relay or fuel
valve solenoid would be placed in that rung of the circuit to be energized when all the
permissive contacts were "good:" that is, all closed. If any one of the permissive
conditions are not met, the series string of switch contacts will be broken, CR2 will de-
energize, and the red lamp will light.

Note that the high fuel pressure contact is normally-closed. This is because we want the
switch contact to open if the fuel pressure gets too high. Since the "normal" condition of
any pressure switch is when zero (low) pressure is being applied to it, and we want this
switch to open with excessive (high) pressure, we must choose a switch that is closed in
its normal state.

Another practical application of relay logic is in control systems where we want to ensure
two incompatible events cannot occur at the same time. An example of this is in
reversible motor control, where two motor contactors are wired to switch polarity (or
phase sequence) to an electric motor, and we don't want the forward and reverse
contactors energized simultaneously:

When contactor M1 is energized, the 3 phases (A, B, and C) are connected directly to
terminals 1, 2, and 3 of the motor, respectively. However, when contactor M2 is
energized, phases A and B are reversed, A going to motor terminal 2 and B going to
motor terminal 1. This reversal of phase wires results in the motor spinning the opposite
direction. Let's examine the control circuit for these two contactors:

Take note of the normally-closed "OL" contact, which is the thermal overload contact
activated by the "heater" elements wired in series with each phase of the AC motor. If the
heaters get too hot, the contact will change from its normal (closed) state to being open,
which will prevent either contactor from energizing.

This control system will work fine, so long as no one pushes both buttons at the same
time. If someone were to do that, phases A and B would be short-circuited together by
virtue of the fact that contactor M1 sends phases A and B straight to the motor and

contactor M2 reverses them; phase A would be shorted to phase B and visa-versa.
Obviously, this is a bad control system design!

To prevent this occurrence from happening, we can design the circuit so that the
energization of one contactor prevents the energization of the other. This is called
interlocking, and it is accomplished through the use of auxiliary contacts on each
contactor, as such:

Now, when M1 is energized, the normally-closed auxiliary contact on the second rung
will be open, thus preventing M2 from being energized, even if the "Reverse" pushbutton
is actuated. Likewise, M1's energization is prevented when M2 is energized. Note, as well,
how additional wire numbers (4 and 5) were added to reflect the wiring changes.

It should be noted that this is not the only way to interlock contactors to prevent a short-
circuit condition. Some contactors come equipped with the option of a mechanical
interlock: a lever joining the armatures of two contactors together so that they are
physically prevented from simultaneous closure. For additional safety, electrical
interlocks may still be used, and due to the simplicity of the circuit there is no good
reason not to employ them in addition to mechanical interlocks.

• REVIEW:
• Switch contacts installed in a rung of ladder logic designed to interrupt a circuit if
certain physical conditions are not met are called permissive contacts, because the
system requires permission from these inputs to activate.
• Switch contacts designed to prevent a control system from taking two
incompatible actions at once (such as powering an electric motor forward and
backward simultaneously) are called interlocks.

Motor control circuits
The interlock contacts installed in the previous section's motor control circuit work fine,
but the motor will run only as long as each pushbutton switch is held down. If we wanted
to keep the motor running even after the operator takes his or her hand off the control

switch(es), we could change the circuit in a couple of different ways: we could replace
the pushbutton switches with toggle switches, or we could add some more relay logic to
"latch" the control circuit with a single, momentary actuation of either switch. Let's see
how the second approach is implemented, since it is commonly used in industry:

When the "Forward" pushbutton is actuated, M1 will energize, closing the normally-open
auxiliary contact in parallel with that switch. When the pushbutton is released, the closed
M1 auxiliary contact will maintain current to the coil of M1, thus latching the "Forward"
circuit in the "on" state. The same sort of thing will happen when the "Reverse"
pushbutton is pressed. These parallel auxiliary contacts are sometimes referred to as seal-
in contacts, the word "seal" meaning essentially the same thing as the word latch.

However, this creates a new problem: how to stop the motor! As the circuit exists right
now, the motor will run either forward or backward once the corresponding pushbutton
switch is pressed, and will continue to run as long as there is power. To stop either circuit
(forward or backward), we require some means for the operator to interrupt power to the
motor contactors. We'll call this new switch, Stop:

Now, if either forward or reverse circuits are latched, they may be "unlatched" by
momentarily pressing the "Stop" pushbutton, which will open either forward or reverse
circuit, de-energizing the energized contactor, and returning the seal-in contact to its
normal (open) state. The "Stop" switch, having normally-closed contacts, will conduct
power to either forward or reverse circuits when released.

So far, so good. Let's consider another practical aspect of our motor control scheme
before we quit adding to it. If our hypothetical motor turned a mechanical load with a lot
of momentum, such as a large air fan, the motor might continue to coast for a substantial
amount of time after the stop button had been pressed. This could be problematic if an
operator were to try to reverse the motor direction without waiting for the fan to stop
turning. If the fan was still coasting forward and the "Reverse" pushbutton was pressed,
the motor would struggle to overcome that inertia of the large fan as it tried to begin
turning in reverse, drawing excessive current and potentially reducing the life of the
motor, drive mechanisms, and fan. What we might like to have is some kind of a time-
delay function in this motor control system to prevent such a premature startup from
happening.

Let's begin by adding a couple of time-delay relay coils, one in parallel with each motor
contactor coil. If we use contacts that delay returning to their normal state, these relays
will provide us a "memory" of which direction the motor was last powered to turn. What
we want each time-delay contact to do is to open the starting-switch leg of the opposite
rotation circuit for several seconds, while the fan coasts to a halt.

If the motor has been running in the forward direction, both M1 and TD1 will have been
energized. This being the case, the normally-closed, timed-closed contact of TD1 between
wires 8 and 5 will have immediately opened the moment TD1 was energized. When the
stop button is pressed, contact TD1 waits for the specified amount of time before
returning to its normally-closed state, thus holding the reverse pushbutton circuit open for
the duration so M2 can't be energized. When TD1 times out, the contact will close and the
circuit will allow M2 to be energized, if the reverse pushbutton is pressed. In like manner,
TD2 will prevent the "Forward" pushbutton from energizing M1 until the prescribed time
delay after M2 (and TD2) have been de-energized.

The careful observer will notice that the time-interlocking functions of TD1 and TD2
render the M1 and M2 interlocking contacts redundant. We can get rid of auxiliary
contacts M1 and M2 for interlocks and just use TD1 and TD2's contacts, since they
immediately open when their respective relay coils are energized, thus "locking out" one
contactor if the other is energized. Each time delay relay will serve a dual purpose:
preventing the other contactor from energizing while the motor is running, and
preventing the same contactor from energizing until a prescribed time after motor
shutdown. The resulting circuit has the advantage of being simpler than the previous
example:

• REVIEW:
• Motor contactor (or "starter") coils are typically designated by the letter "M" in
ladder logic diagrams.
• Continuous motor operation with a momentary "start" switch is possible if a
normally-open "seal-in" contact from the contactor is connected in parallel with
the start switch, so that once the contactor is energized it maintains power to itself
and keeps itself "latched" on.
• Time delay relays are commonly used in large motor control circuits to prevent
the motor from being started (or reversed) until a certain amount of time has
elapsed from an event.

Fail-safe design
Logic circuits, whether comprised of electromechanical relays or solid-state gates, can be
built in many different ways to perform the same functions. There is usually no one
"correct" way to design a complex logic circuit, but there are usually ways that are better
than others.

In control systems, safety is (or at least should be) an important design priority. If there
are multiple ways in which a digital control circuit can be designed to perform a task, and
one of those ways happens to hold certain advantages in safety over the others, then that
design is the better one to choose.

Let's take a look at a simple system and consider how it might be implemented in relay
logic. Suppose that a large laboratory or industrial building is to be equipped with a fire

alarm system, activated by any one of several latching switches installed throughout the
facility. The system should work so that the alarm siren will energize if any one of the
switches is actuated. At first glance it seems as though the relay logic should be
incredibly simple: just use normally-open switch contacts and connect them all in parallel
with each other:

Essentially, this is the OR logic function implemented with four switch inputs. We could
expand this circuit to include any number of switch inputs, each new switch being added
to the parallel network, but I'll limit it to four in this example to keep things simple. At
any rate, it is an elementary system and there seems to be little possibility of trouble.

Except in the event of a wiring failure, that is. The nature of electric circuits is such that
"open" failures (open switch contacts, broken wire connections, open relay coils, blown
fuses, etc.) are statistically more likely to occur than any other type of failure. With that
in mind, it makes sense to engineer a circuit to be as tolerant as possible to such a failure.
Let's suppose that a wire connection for Switch #2 were to fail open:

If this failure were to occur, the result would be that Switch #2 would no longer energize
the siren if actuated. This, obviously, is not good in a fire alarm system. Unless the
system were regularly tested (a good idea anyway), no one would know there was a
problem until someone tried to use that switch in an emergency.

What if the system were re-engineered so as to sound the alarm in the event of an open
failure? That way, a failure in the wiring would result in a false alarm, a scenario much
more preferable than that of having a switch silently fail and not function when needed.
In order to achieve this design goal, we would have to re-wire the switches so that an
open contact sounded the alarm, rather than a closed contact. That being the case, the
switches will have to be normally-closed and in series with each other, powering a relay
coil which then activates a normally-closed contact for the siren:

When all switches are unactuated (the regular operating state of this system), relay CR1
will be energized, thus keeping contact CR1 open, preventing the siren from being
powered. However, if any of the switches are actuated, relay CR1 will de-energize,
closing contact CR1 and sounding the alarm. Also, if there is a break in the wiring

anywhere in the top rung of the circuit, the alarm will sound. When it is discovered that
the alarm is false, the workers in the facility will know that something failed in the alarm
system and that it needs to be repaired.

Granted, the circuit is more complex than it was before the addition of the control relay,
and the system could still fail in the "silent" mode with a broken connection in the bottom
rung, but it's still a safer design than the original circuit, and thus preferable from the
standpoint of safety.

This design of circuit is referred to as fail-safe, due to its intended design to default to the
safest mode in the event of a common failure such as a broken connection in the switch
wiring. Fail-safe design always starts with an assumption as to the most likely kind of
wiring or component failure, and then tries to configure things so that such a failure will
cause the circuit to act in the safest way, the "safest way" being determined by the
physical characteristics of the process.

Take for example an electrically-actuated (solenoid) valve for turning on cooling water to
a machine. Energizing the solenoid coil will move an armature which then either opens or
closes the valve mechanism, depending on what kind of valve we specify. A spring will
return the valve to its "normal" position when the solenoid is de-energized. We already
know that an open failure in the wiring or solenoid coil is more likely than a short or any
other type of failure, so we should design this system to be in its safest mode with the
solenoid de-energized.

If it's cooling water we're controlling with this valve, chances are it is safer to have the
cooling water turn on in the event of a failure than to shut off, the consequences of a
machine running without coolant usually being severe. This means we should specify a
valve that turns on (opens up) when de-energized and turns off (closes down) when
energized. This may seem "backwards" to have the valve set up this way, but it will make
for a safer system in the end.

One interesting application of fail-safe design is in the power generation and distribution
industry, where large circuit breakers need to be opened and closed by electrical control
signals from protective relays. If a 50/51 relay (instantaneous and time overcurrent) is
going to command a circuit breaker to trip (open) in the event of excessive current,
should we design it so that the relay closes a switch contact to send a "trip" signal to the
breaker, or opens a switch contact to interrupt a regularly "on" signal to initiate a breaker
trip? We know that an open connection will be the most likely to occur, but what is the
safest state of the system: breaker open or breaker closed?

At first, it would seem that it would be safer to have a large circuit breaker trip (open up
and shut off power) in the event of an open fault in the protective relay control circuit,
just like we had the fire alarm system default to an alarm state with any switch or wiring
failure. However, things are not so simple in the world of high power. To have a large
circuit breaker indiscriminately trip open is no small matter, especially when customers
are depending on the continued supply of electric power to supply hospitals,

telecommunications systems, water treatment systems, and other important
infrastructures. For this reason, power system engineers have generally agreed to design
protective relay circuits to output a closed contact signal (power applied) to open large
circuit breakers, meaning that any open failure in the control wiring will go unnoticed,
simply leaving the breaker in the status quo position.

Is this an ideal situation? Of course not. If a protective relay detects an overcurrent
condition while the control wiring is failed open, it will not be able to trip open the circuit
breaker. Like the first fire alarm system design, the "silent" failure will be evident only
when the system is needed. However, to engineer the control circuitry the other way -- so
that any open failure would immediately shut the circuit breaker off, potentially blacking
out large potions of the power grid -- really isn't a better alternative.

An entire book could be written on the principles and practices of good fail-safe system
design. At least here, you know a couple of the fundamentals: that wiring tends to fail
open more often than shorted, and that an electrical control system's (open) failure mode
should be such that it indicates and/or actuates the real-life process in the safest
alternative mode. These fundamental principles extend to non-electrical systems as well:
identify the most common mode of failure, then engineer the system so that the probable
failure mode places the system in the safest condition.

• REVIEW:
• The goal of fail-safe design is to make a control system as tolerant as possible to
likely wiring or component failures.
• The most common type of wiring and component failure is an "open" circuit, or
broken connection. Therefore, a fail-safe system should be designed to default to
its safest mode of operation in the case of an open circuit.

Programmable logic controllers
Before the advent of solid-state logic circuits, logical control systems were designed and
built exclusively around electromechanical relays. Relays are far from obsolete in modern
design, but have been replaced in many of their former roles as logic-level control
devices, relegated most often to those applications demanding high current and/or high
voltage switching.

Systems and processes requiring "on/off" control abound in modern commerce and
industry, but such control systems are rarely built from either electromechanical relays or
discrete logic gates. Instead, digital computers fill the need, which may be programmed
to do a variety of logical functions.

In the late 1960's an American company named Bedford Associates released a computing
device they called the MODICON. As an acronym, it meant Modular Digital Controller,
and later became the name of a company division devoted to the design, manufacture,

and sale of these special-purpose control computers. Other engineering firms developed
their own versions of this device, and it eventually came to be known in non-proprietary
terms as a PLC, or Programmable Logic Controller. The purpose of a PLC was to
directly replace electromechanical relays as logic elements, substituting instead a solid-
state digital computer with a stored program, able to emulate the interconnection of many
relays to perform certain logical tasks.

A PLC has many "input" terminals, through which it interprets "high" and "low" logical
states from sensors and switches. It also has many output terminals, through which it
outputs "high" and "low" signals to power lights, solenoids, contactors, small motors, and
other devices lending themselves to on/off control. In an effort to make PLCs easy to
program, their programming language was designed to resemble ladder logic diagrams.
Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic
schematics would feel comfortable programming a PLC to perform the same control
functions.

PLCs are industrial computers, and as such their input and output signals are typically
120 volts AC, just like the electromechanical control relays they were designed to
replace. Although some PLCs have the ability to input and output low-level DC voltage
signals of the magnitude used in logic gate circuits, this is the exception and not the rule.

Signal connection and programming standards vary somewhat between different models
of PLC, but they are similar enough to allow a "generic" introduction to PLC
programming here. The following illustration shows a simple PLC, as it might appear
from a front view. Two screw terminals provide connection to 120 volts AC for powering
the PLC's internal circuitry, labeled L1 and L2. Six screw terminals on the left-hand side
provide connection to input devices, each terminal representing a different input
"channel" with its own "X" label. The lower-left screw terminal is a "Common"
connection, which is generally connected to L2 (neutral) of the 120 VAC power source.

Inside the PLC housing, connected between each input terminal and the Common
terminal, is an opto-isolator device (Light-Emitting Diode) that provides an electrically

isolated "high" logic signal to the computer's circuitry (a photo-transistor interprets the
LED's light) when there is 120 VAC power applied between the respective input terminal
and the Common terminal. An indicating LED on the front panel of the PLC gives visual
indication of an "energized" input:

Output signals are generated by the PLC's computer circuitry activating a switching
device (transistor, TRIAC, or even an electromechanical relay), connecting the "Source"
terminal to any of the "Y-" labeled output terminals. The "Source" terminal,
correspondingly, is usually connected to the L1 side of the 120 VAC power source. As
with each input, an indicating LED on the front panel of the PLC gives visual indication
of an "energized" output:

In this way, the PLC is able to interface with real-world devices such as switches and
solenoids.

The actual logic of the control system is established inside the PLC by means of a
computer program. This program dictates which output gets energized under which input
conditions. Although the program itself appears to be a ladder logic diagram, with switch
and relay symbols, there are no actual switch contacts or relay coils operating inside the
PLC to create the logical relationships between input and output. These are imaginary
contacts and coils, if you will. The program is entered and viewed via a personal
computer connected to the PLC's programming port.

Consider the following circuit and PLC program:

When the pushbutton switch is unactuated (unpressed), no power is sent to the X1 input
of the PLC. Following the program, which shows a normally-open X1 contact in series
with a Y1 coil, no "power" will be sent to the Y1 coil. Thus, the PLC's Y1 output remains
de-energized, and the indicator lamp connected to it remains dark.

If the pushbutton switch is pressed, however, power will be sent to the PLC's X1 input.
Any and all X1 contacts appearing in the program will assume the actuated (non-normal)
state, as though they were relay contacts actuated by the energizing of a relay coil named
"X1". In this case, energizing the X1 input will cause the normally-open X1 contact will
"close," sending "power" to the Y1 coil. When the Y1 coil of the program "energizes,"
the real Y1 output will become energized, lighting up the lamp connected to it:

It must be understood that the X1 contact, Y1 coil, connecting wires, and "power"
appearing in the personal computer's display are all virtual. They do not exist as real
electrical components. They exist as commands in a computer program -- a piece of
software only -- that just happens to resemble a real relay schematic diagram.

Equally important to understand is that the personal computer used to display and edit the
PLC's program is not necessary for the PLC's continued operation. Once a program has
been loaded to the PLC from the personal computer, the personal computer may be
unplugged from the PLC, and the PLC will continue to follow the programmed
commands. I include the personal computer display in these illustrations for your sake
only, in aiding to understand the relationship between real-life conditions (switch closure
and lamp status) and the program's status ("power" through virtual contacts and virtual
coils).

The true power and versatility of a PLC is revealed when we want to alter the behavior of
a control system. Since the PLC is a programmable device, we can alter its behavior by
changing the commands we give it, without having to reconfigure the electrical
components connected to it. For example, suppose we wanted to make this switch-and-
lamp circuit function in an inverted fashion: push the button to make the lamp turn off,
and release it to make it turn on. The "hardware" solution would require that a normally-
closed pushbutton switch be substituted for the normally-open switch currently in place.
The "software" solution is much easier: just alter the program so that contact X1 is
normally-closed rather than normally-open.

In the following illustration, we have the altered system shown in the state where the
pushbutton is unactuated (not being pressed):

In this next illustration, the switch is shown actuated (pressed):

One of the advantages of implementing logical control in software rather than in
hardware is that input signals can be re-used as many times in the program as is
necessary. For example, take the following circuit and program, designed to energize the
lamp if at least two of the three pushbutton switches are simultaneously actuated:

To build an equivalent circuit using electromechanical relays, three relays with two
normally-open contacts each would have to be used, to provide two contacts per input
switch. Using a PLC, however, we can program as many contacts as we wish for each
"X" input without adding additional hardware, since each input and each output is
nothing more than a single bit in the PLC's digital memory (either 0 or 1), and can be
recalled as many times as necessary.

Furthermore, since each output in the PLC is nothing more than a bit in its memory as
well, we can assign contacts in a PLC program "actuated" by an output (Y) status. Take
for instance this next system, a motor start-stop control circuit:

The pushbutton switch connected to input X1 serves as the "Start" switch, while the
switch connected to input X2 serves as the "Stop." Another contact in the program,
named Y1, uses the output coil status as a seal-in contact, directly, so that the motor
contactor will continue to be energized after the "Start" pushbutton switch is released.
You can see the normally-closed contact X2 appear in a colored block, showing that it is
in a closed ("electrically conducting") state.

If we were to press the "Start" button, input X1 would energize, thus "closing" the X1
contact in the program, sending "power" to the Y1 "coil," energizing the Y1 output and
applying 120 volt AC power to the real motor contactor coil. The parallel Y1 contact will
also "close," thus latching the "circuit" in an energized state:

Now, if we release the "Start" pushbutton, the normally-open X1 "contact" will return to
its "open" state, but the motor will continue to run because the Y1 seal-in "contact"
continues to provide "continuity" to "power" coil Y1, thus keeping the Y1 output
energized:

To stop the motor, we must momentarily press the "Stop" pushbutton, which will
energize the X2 input and "open" the normally-closed "contact," breaking continuity to
the Y1 "coil:"

When the "Stop" pushbutton is released, input X2 will de-energize, returning "contact"
X2 to its normal, "closed" state. The motor, however, will not start again until the "Start"
pushbutton is actuated, because the "seal-in" of Y1 has been lost:

An important point to make here is that fail-safe design is just as important in PLC-
controlled systems as it is in electromechanical relay-controlled systems. One should
always consider the effects of failed (open) wiring on the device or devices being
controlled. In this motor control circuit example, we have a problem: if the input wiring
for X2 (the "Stop" switch) were to fail open, there would be no way to stop the motor!

The solution to this problem is a reversal of logic between the X2 "contact" inside the
PLC program and the actual "Stop" pushbutton switch:

When the normally-closed "Stop" pushbutton switch is unactuated (not pressed), the
PLC's X2 input will be energized, thus "closing" the X2 "contact" inside the program.
This allows the motor to be started when input X1 is energized, and allows it to continue
to run when the "Start" pushbutton is no longer pressed. When the "Stop" pushbutton is
actuated, input X2 will de-energize, thus "opening" the X2 "contact" inside the PLC
program and shutting off the motor. So, we see there is no operational difference between
this new design and the previous design.

However, if the input wiring on input X2 were to fail open, X2 input would de-energize
in the same manner as when the "Stop" pushbutton is pressed. The result, then, for a
wiring failure on the X2 input is that the motor will immediately shut off. This is a safer
design than the one previously shown, where a "Stop" switch wiring failure would have
resulted in an inability to turn off the motor.

In addition to input (X) and output (Y) program elements, PLCs provide "internal" coils
and contacts with no intrinsic connection to the outside world. These are used much the
same as "control relays" (CR1, CR2, etc.) are used in standard relay circuits: to provide
logic signal inversion when necessary.

To demonstrate how one of these "internal" relays might be used, consider the following
example circuit and program, designed to emulate the function of a three-input NAND
gate. Since PLC program elements are typically designed by single letters, I will call the
internal control relay "C1" rather than "CR1" as would be customary in a relay control
circuit:

In this circuit, the lamp will remain lit so long as any of the pushbuttons remain
unactuated (unpressed). To make the lamp turn off, we will have to actuate (press) all
three switches, like this:

This section on programmable logic controllers illustrates just a small sample of their
capabilities. As computers, PLCs can perform timing functions (for the equivalent of
time-delay relays), drum sequencing, and other advanced functions with far greater
accuracy and reliability than what is possible using electromechanical logic devices. Most
PLCs have the capacity for far more than six inputs and six outputs. The following
photograph shows several input and output modules of a single Allen-Bradley PLC.

With each module having sixteen "points" of either input or output, this PLC has the
ability to monitor and control dozens of devices. Fit into a control cabinet, a PLC takes
up little room, especially considering the equivalent space that would be needed by
electromechanical relays to perform the same functions:

One advantage of PLCs that simply cannot be duplicated by electromechanical relays is
remote monitoring and control via digital computer networks. Because a PLC is nothing
more than a special-purpose digital computer, it has the ability to communicate with other
computers rather easily. The following photograph shows a personal computer displaying
a graphic image of a real liquid-level process (a pumping, or "lift," station for a municipal
wastewater treatment system) controlled by a PLC. The actual pumping station is located
miles away from the personal computer display:

Ladder logic tutorial

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