PLC Basics: The Ultimate Guide in 2026 - PLCGurus.NET
By PLCGuru
Published: 2025-03-12 · Archived: 2026-04-05 17:10:09 UTC
We are often discussing more advanced topics here at PLCGurus.NET for the more seasoned programmers. This
article aims to change that! Operator training is crucial for the effective use and maintenance of PLC systems.
Targeting foundational Programmable Logic Controller skills (PLC Basics), this article will serve as a starting
point for those individuals looking to enter into the fascinating world of PLCs, Automation and Control.
This article will cover PLC basics in the following topic areas:
What is a PLC?
Why You Should Use a PLC
How Do PLCs Work?
Fundamental Difference Between a PLC and PC
Working With Number Systems
Difference Between Discrete and Analog I/O?
Logic Gates & Boolean Expressions
Ladder Logic  Programming
Function Block Programming
Sequential Text Programming
Considerations When Choosing a PLC
The complexity of control processes plays a crucial role in determining the type of PLC needed, with simpler
processes requiring basic controls and more intricate operations necessitating advanced capabilities.
If you’re new to  programmable logic control and the world of industrial automation, then join us as we explore
each one of these topics at length.
Programming
PLC Basics – What Is A PLC?
The term PLC stands for Programmable Logic Controller. The PLC was invented by a young engineer, Dick
Morley, back in 1964. Since this time, the PLC has revolutionized the industrial and manufacturing landscape to
become arguably the most integral component of any industrial process in existence today. PLCs have
revolutionized industrial automation by replacing complex relay-based control systems with easier software-based
logic.
Initially the PLC was used to replace relay logic, but today its range of functions includes timing, counting,
calculating, comparing, and the processing of analog signals.
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A PLC is known as a “hard real-time system” due to the fact that outputs must respond to changing inputs within a
very stringent time constraint. That said, you can think of a PLC as nothing more than a dedicated  computer
designed for fast switching and decision making.
The main advantage of a PLC over a “hard-wired” type relay control systems, is that PLCs lend themselves to
functional changes very easily. What do I mean by this you ask? PLCs simplify system updates and maintenance
by eliminating the need for rewiring when modifying control logic.
A Simple Example
Imagine you have a switch that turns on a light. The light has two states, ON or OFF, and will respond almost
instantaneously to someone either flipping the switch ON, or OFF.
Now imagine you wire this switch up directly to the light and your boss comes to tell you that he would like the
light to come on precisely 30 seconds after the switch turns on…you have a problem! In order to achieve this it
will require additional hardware, i.e., a timing relay, and some rewiring. Now enter the PLC!
With a PLC there will be no need to buy additional hardware every time a change is required. All that will be
needed is a simple programming change that will delay the light from turning on until 30 seconds after the switch
is thrown.
Computer Hardware
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As you can see the light switch becomes an “input” to the PLC, and the light itself is an “output”.
Granted, this is a very simple example, however, even larger more complex systems are nothing more than a
combination of switch inputs and an assortment of outputs. Then by using software we can build logic to control
the outputs based in the input conditions. The PLC sends control signals to manage the light, ensuring it turns on
30 seconds after the switch is activated.
PLC Basics – How Do PLCs Work?
Programmable logic controllers are specialized computers designed to run manufacturing processes. The structure
of a PLC is based on the same principles as those employed in computer architectures. You can think of a PLC as
a highly “ruggedized” industrial computer that is designed to withstand harsh environments (i.e., high
temperatures, dirty or dusty environments) and is highly modular.
This means that you can add various Input/Output modules and module types in an almost unlimited ordering –
the only constraint being the number of Input/Output slots you have available in your physical rack (chassis). The
main components of a PLC are:
Rack or Chassis
Power Supply
Central Processing Unit (CPU)
Communication Modules
Inputs and Outputs
Digital inputs play a crucial role in receiving and processing signals from various sensors and switches, converting
real-world signals into binary data that the  CPU can interpret and act upon. Input devices transmit signals to
the PLC from sensors and switches that monitor environmental changes.
PLCs come in all shapes and sizes depending on the process you are intending to control, the memory and
Input/Output (I/O) requirements.
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PLC Basics – Rack (Chassis)
The PLC rack (also known as “chassis”) is the backbone of all PLC systems. The chassis provides all the
necessary mounting for the power supply, backplane and all I/O modules that will reside in it. Illustrated in the
images below is an example of an Allen-Bradley ControlLogix 1756-A7 chassis.
Electronics & Electrical
As you can see in this image that aside from the power supply unit located to the far left of the chassis, the rack is
essentially empty.
PLC Basics – Power Supply and Backplane
The power supply provides all the necessary voltages to the “backplane” of the PLC chassis. Typical voltages will
be 24 VDC and 5 VDC to power the CPU, Communication, and Input/Output Modules.
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PLC Basics – Central Processing Unit (CPU)
The  central processing unit or CPU is the main “brain” of the PLC. The memory structure of a PLC processor
consists of several areas, some of these having specific roles.
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With “rack-based” memory structure addresses are derived using the rack number, the I/O module slot number
and the screw terminal number where the I/O device is wired into.
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A typical “rack-based” PLC is the SLC 500 platform of  programmable logic controllers. Their memory space
is divided into two broad categories, namely, Program Files and Data Files.
With “tag-based” memory structures all data are assigned a variable name called a “tag”. A program can be
developed using only tag names but you must assign input and output tags before the program can be executed.
The ControlLogix platform of programmable logic controllers employ “tag-based” memory structures.
A PLC program enhances the efficiency and functionality of the system by allowing for complex applications and
simplifying the programming process, reducing the need for extensive wiring and making system modifications
easier.
Although the image above by convention is residing in Slot 0, with the Logix platform of controllers this is not
mandatory as it was in previous platforms of PLC such as the SLC 500 family of controllers.
PLC Basics – Communication Modules
Communication modules allow the PLC to “talk” over various network protocols. Common modern network
protocols used are ControlNet, DeviceNet, and Ethernet/IP. These modules will allow the CPU to communicate to
other PLC controllers and/or Remote I/O racks that are distributed in the field.
The advantage of Remote I/O (I/O that is field distributed) versus Local I/O (these are input and output modules
that reside in the same rack as the CPU), is that it saves on installation time and money. Rather than having to run
the wires for all your field input and output devices back to the panel where the local PLC resides, you can “drop”
a Remote I/O rack in close proximity to the field I/O devices and wire them up locally.
After the remote rack is wired up in the field, you merely need to run a communication cable, i.e., Ethernet cable,
and possibly a couple of low voltage power conductors to power the remote I/O communication adapter its I/O.
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Communication modules send control signals to connected output devices like motors and valves, ensuring
efficient operation of the industrial process. They provide installation flexibility and data acquisition capabilities.
PLC Basics – Modes Of Operation
A PLC has basically two modes of operation: the Program Mode and some variation of the Run Mode. A three-position keyswitch may be used to select different processor modes of operation.
Program Mode is used to enter a new program, edit or update an existing program, upload files and
download files. It is important to note that in this mode of operation all outputs are de-energized.
Run Mode is used to execute the user program. Remote PLC programming or mode selection is disabled
when the key is in the Run Mode position.
Remote Run Mode (REM) allows the PLC to be remotely changed between program and run mode by a
personal  computer connected either directly or via a communication protocol to the PLC processor.
Typically most processors are placed into REM mode to allow the engineering or maintenance staff the
greatest flexibility when performing PLC programming tasks.
PLC Basics – Understanding CPU Scan
Very simply, during each program scan cycle the processor reads all the inputs, takes these value, and energizes or
de-energizes the outputs according to the user program.
The Program Scan Cylce looks like this:
1. Scan Inputs – is the input ON (1) or OFF (0).
2. Execute Program Logic – executing each instruction and solve the rung logic.
3. Update Outputs – write a logic 1 (ON) or 0 (OFF) to the output.
4. House-Keeping – perform internal checks and system tasks.
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Granted this is a bit of a simplification and with more modern PLC’s or PAC’s (  Programmable Automation
Controllers) as they’re commonly referred, more elaborate scan patterns can be configured. But I say let’s not
muddy the waters too much here.
If you are interested in a sneak peak at one of the videos in our Studio 5000 Essentials video series where we
discuss more advanced scan patterns, go ahead and view it now!
Not to oversimplify the importance of processor scan and its impacts on overall response time, however, since this
is an introductory welcome to PLC Basics article.
I will reserve those more advanced discussions for other articles. In fact, we’ve done an article on this very topic
at System Overhead Time Slice.
PLC Basics – Difference Between a PLC and PC
As I mentioned the architecture of a PLC is basically the same as that of a personal computer. However, unlike
PC’s the PLC is designed to operate in the harshest of industrial environments that have a wide range of ambient
temperatures and humidity. Additionally, properly designed PLC installations can mitigate EMI (electro-magnetic
interferance – NOISE) present in almost all industrial establishments.
PC’s are highly complex computing machines capable of executing several programs and tasks concurrently
whereas a PLC is a dedicated real-time system that executes a single (can have multiple programs in ControlLogix
system, however, they still executed one at a time in a prioritized ordering) program in an orderly and sequential
fashion from first to last instruction.
Unlike PC’s, the PLC is programmed in relay ladder logic or other “easily” learned languages. It comes with its
 programming language built into its memory and has no permanently attached keyboard, monitor, CD drive,
printer etc.
PLC control systems have been designed with maintainability and ease of installation as a key factor.
Troubleshooting is simplified by the use of fault indicators on the processor and I/O modules. Furthermore, in
traditional PLC chassis I/O is modularized to allow easy replacement and configuration.
PLC Basics – Knowing Your Bits and Bytes!
It is important that you understand how PLC memory is arranged. In its most basic form, a piece of data is stored
as either a 0 or a 1 in what’s referred to as a “Bit” of memory.
When 4-bits are stored in contiguous memory it is referred to as a “Nibble“. When 8-bits are stored in contiguous
memory it is referred to as a “Byte“.
Therefore, 1 Byte = 2 Nibbles = 8 Bits. Expanding on this concept, when 2-bytes are stored in contiguous
memory it is referred to as a “Word“.
Therefore, 1 Word = 2 Bytes = 16 Bits. When you hear that a given PLC or computer for that matter is an 8, 16,
32, 64-bit architecture, it is referring to the number of bits allocated in each contiguous memory location.
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This means that each memory location in a 16-bit architecture, such as the Allen-Bradley SLC 500 platform of
controllers, has 16-bit words, or can represent a signed integer range of -32,768 to 32,767.
With advancements in  programmable logic controllers as of late, memory now supports 32-bit architectures.
This means that each memory location has 32-bits, which is referred to a double word or “DWord“.
Therefore, 1 DWord = 2 Words = 4 Bytes = 32 Bits. A 32-bit architecture can represent a signed integer range
of ?2,147,483,648 to 2,147,483,647.
The image below captures everything we’ve discussed here in visual form:
Now, if we expand this concept to the modern day PC’s and laptops that boast a 64-bit architecture, also referred
to as a quad-word or “QWord“, what is the largest signed integer that we can represent with that??? I’ll leave it to
you to research that, but let’s just say it’s a really, really big number!
PLC Basics – Working With Number Systems
In order to be proficient as a PLC programmer it is imperative that you are comfortable moving in and out of
different numbering systems
By far the most important number systems that you will need to have command over is the binary number system
(base 2), or the number system that contains only 0’s and 1’s. However, there are other number systems we must
be comfortable moving in and out of as well, namely, the hexadecimal system (base 16) and the octal system (base
8), and of course the decimal system (base 10), but I’m going to assume you’re okay with that one!
If you’re unclear why I’ve included the “base x” in each number system, it’s because it gives us an indication of
the permissible numbers in that system. What do I mean?
For example, the decimal (base 10) system that we use every day contains valid numbers, 0..9. In general, we can
say that for a give number system with a base n, there is 0..n-1 numbers we can use in that system – with one
exception the hexadecimal system.
The Binary System (base 2) has valid numbers, 0..1
The Octal System (base 8) had valid numbers, 0..7
The Hexadecimal System (base 16) has valid numbers, 0..9 and A..F (more on this later)
PLC’s like PC’s can only interpret 0’s (low signals) and 1’s (high signals) because it is made up of millions of tiny
transistors that act as switches being driven by high and low electrical signals. This reminds me of a funny joke I
once heard,
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There are 3 types of people in the world…those who understand binary, and those who don’t!
Alright, maybe not the best joke…but it’s a little funny right? Anyhow, knowing the binary number system and
these other systems is essential to our understanding of programmable logic control.
PLC Basics – The Binary System
Let’s start by looking at the system we are most comfortable with, the decimal system. If we take the following
number: 97510
What is this number in reality? We said that the decimal system is base 10, so to compute the number 97510 it is
equal to the following:
Notice the “10” subscript. We usually indicate the base of the number by including the subscript as shown. Let’s
try some more:
Are you getting the gist of it? The base of the number system represents the multiplier raised to the exponent “x”
depending on the position of the digit. Let’s try some binary!
Remember we said that the binary number system is base 2. This means that the multiplier is 2 raised to the
exponent “x” depending on the position of the digit. Let’s try some:
Let’s try one that’s a little harder:
So this shows us a nice convenient way to convert a binary number to a decimal number, but how about the other
way around? What if we need to convert a decimal number to a binary number??? We must divide and conquer!
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To convert a decimal number to a binary number we must divide by the base of the number we are converting to.
In the case of binary this of course is 2. The remainder, either a 0 or 1 becomes the number in the binary sequence.
Let’s try one!
Convert 97610 to binary. Start by dividing the initial number (976) by 2, then continue dividing the resultant
number by 2 until the result is 0.
Since the result is 0, we are done!
The key is to NOT to read the binary string top to bottom, but to read the resultant binary string bottom to top.
Therefore the correct answer: 97610 = 11110100002
PLC Basics – The Octal System
The octal system is quickly disappearing, however, if you encounter an 8-bit architecture PLC such as the Allen-Bradley PLC 5, then knowing octal will be an asset.
Luckily what we’ve learned so far is going to serve us well here too. As discussed the octal system is a base 8
number system. This means that permissible numbers will be between 0..7.
To convert a binary number to octal requires a 2-step process. First, convert the binary number to it decimal (base
10) equivalent, then using the same “divide and conquer” technique used above, convert the decimal equivalent to
octal. Only this time, instead of dividing by 2 we will be dividing by 8.
Let’s use the same example we did above and convert 11012 to its octal equivalent. Therefore,
Now we divide by 8 to find the octal equivalent,
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As we did before, since the result is 0 we need not perform any more operations and we read the resultant octal
string top to bottom.
Therefore, 11012 = 158
This same algorithm can be used to compute more complex binary conversions to octal strings.
PLC Basics – The Hexadecimal System
Next to binary, hexadecimal is probably the next most important number system you should be comfortable with.
The hexadecimal system uses a base 16 number system, with integer values 0..9 and letter A=10, B=11, C=12,
D=13, E=14 and F=15.
Hexadecimal numbers are commonly used in computing because they can express every byte as two consecutive
hexadecimal digits versus eight bits. This is useful when identifying memory locations and is much more readable
for humans.
For example, the binary byte 111011012 can be expressed in hexadecimal format by separating the binary string
into its 4-bit nibbles. Then convert the 4-bit nibbles into its decimal (base 10) equivalent. Once it’s in its decimal
(base 10) form, convert it to its hexadecimal (base 16) equivalent. Let’s try it!
Convert 111011012 to its hexadecimal equivalent.
First, separate the binary string into its 4-bit nibbles: 1110 1101
Now treat each nibble separately, namely:
High-order nibble: 11102 = 1410 = E16
Low-order nibble: 11012 = 1310 = D16
Therefore, 111011012 = ED16
This can be confirmed using the calculator on your  computer in “Programmer” mode as seen below.
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Because hexadecimal form is used so extensively in PLC’s and computing let’s try a more difficult example and
then verify it using our calculator as we did above.
Convert 11010110101010012 to its hexadecimal equivalent.
First, separate the binary string into its 4-bit nibbles: 1101 0110 1010 1001
Now treat each nibble separately, namely:
11012 = 1310 = D16
01102 = 610 = 616
10102 = 1010 = A16
10012 = 910 = 916
Therefore, 11010110101010012 = D6A916
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PLC Basics – A Quick Word about BCD…
BCD is known as Binary Coded Decimal. It is very similar in structure to the hexadecimal system just discussed
with some one difference. The binary coding limits us to using only number 0 through 9.
This type of encoding was widely used for devices such as Thumbwheel switches, 7-Segment Displays and
Encoders.
We can use the same convention we used to “break-up” a 16-bit binary string for hexadecimal conversion to
convert an binary number to its BCD equivalent.
Example: Convert the binary string 1001011100010011 to BCD format.
First we parse the 16-bit binary string into its 4-bit nibbles as follows,
1001 0111 0001 0011
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Then computing each 4-bit nibble to its decimal equivalent:
1001 = 9
0111 = 7
0001 = 1
0011 = 3
Therefore, the BCD equivalent is 9713. Binary Coded Decimal was created to provide a more human readable
format than hexadecimal by limiting the usable values between 0..9 and omitting the letters A..F.
PLC Basics – Floating Point Numbers
Floating point numbers (also known as “Real” or “Decimal” numbers) give us the ability to represent fractional
numbers with a finite precision. Depending on the architecture of your PLC, i.e., 16-bit or 32-bit, the large the
memory the greater it will allow you to better approximate the fractional number you wish to represent.
Nearly all  computers today follow the the IEEE 754 standard for representing floating-point numbers. This
standard was largely developed by 1980 and it was formally adopted in 1985, though several manufacturers
continued to use their own formats throughout the 1980’s.
It should be noted that some numbers go on to infinity, for example pi. This number never ends, so it should be
clear that while we can approximate these numbers to a high precision, that it is only an approximation.
PLC Basics – Discrete Inputs and Outputs
Discrete Inputs (Digital Inputs)
Discrete Input interface modules connects field input devices of the ON/OFF nature. This classification of I/O is
related to bit oriented inputs and outputs – simply put – they can be described in the controllers memory using a 1
or 0.
Common voltage sources are 120 VAC and more commonly 24 VDC. Discrete PLC modules will specify whether
it will accept AC, DC or both AC and DC, therefore, careful selection is required when sourcing these modules. A
digital input card will receive an electrical signal (high or low) that will be interpreted as either ON or OFF.
Examples of Discrete Inputs are:
Limit switches
Proximity switches
Pushbuttons
Selector Switches
Discrete Outputs (Output Devices)
Discrete Output interface modules connect field output devices of the ON/OFF nature. A digital output module
with either turn a device ON or OFF based on the logic that is controlling it and the input states that it depends on.
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Examples of Discrete Outputs are:
Control Relays
Motor Contactors
Pilot Lights
Solenoid Valves
PLC Basics – Analog Inputs and Outputs
Analog Inputs
Analog Input interface modules convert a voltage or current (e.g. a signal that can be anywhere from 0 to 20mA)
into a digitally equivalent number that can be understood by the  CPU through an Analog- Digital Conversion
(ADC) method known as Quantization. Analog input signals to a PLC can vary continuously over a range of
voltage or current, providing detailed information about connected devices.
To input an analog voltage (into a PLC or any other  computer) the continuous voltage value must be sampled
and then converted to a numerical value by an ADC.
The time required to acquire the sample is called the sampling time. ADCs can only acquire a limited number of
samples per second. The time between samples is called the sampling period, T, and the inverse of the sampling
period is the sampling frequency (also called sampling rate).
The sampling time is often much smaller than the sampling period. The sampling frequency is specified when
buying hardware, but for a PLC a maximum sampling rate might be 20Hz.
Resolution is another term you will often hear in the field when dealing with analog type instruments or sensors.
Resolution is defined as the the smallest signal that can be represented by the ADC, or,
Resolution =  Full Scale Value / 2n
Where n is the number of bits allocated to the conversion, which in most cases will be 16-, or 32-bits depending
on your controller.
Example: If the analog input module you are using has a Full Scale Value = 10V, and n = 16 bits, then,
Resolution  =   10V / 216 = 0.00015258789 V
This means we can be accurate, or detect a change in voltage to within 0.00015258789 V. Examples of Analog
Inputs are:
Linear Variable Displacement Transducers (LVDTs)
Thermocouples
Resistance Temperature Sensors (RTDs)
Flow Sensors
Potentiometers
Analog Outputs
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Analog Output interface modules will convert a digital number sent by the CPU to it’s real world voltage or
current.  Typical outputs signals can range from -10 VDC to +10 VDC, or 0-20mA and are used to drive mass
flow controllers, pressure regulators and position controls.
Analog outputs are much simpler than analog inputs. To set an an analog output an integer is converted to a
voltage. This process is very fast, however, analog outputs are subject to known as quantization errors. Examples
of Analog Outputs are:
Proportional Valves
Servo Motors
Heaters
Analog outputs control various output devices like motors, valves, and heaters, playing an essential role in
managing industrial processes.
PLC Basics – Logic Gates & Boolean Expressions
Logic gates in their basic form are electronic circuits that operate on one or more inputs to produce an output
signal. They are the fundamental building blocks of any digital circuit. Most logic gates will accept two inputs and
determine one output, however there are a few exceptions to this.
Let’s focus our attention on the most common logic gates that will translate directly into PLC  programming
and Ladder Logic.
To evaluate logic gates is sometime useful to use something known as a Truth Table. Truth tables allow us to
evaluate every combination of a logic gate or the combination of many logic gates built into a circuit.
The NOT function
The NOT function is perhaps the simplest of all gates. It’s only purpose is to invert or flip whatever the input
signal is to the output. For example, if the input is 1 the output is 0 and vice versa, if the input is 0 the output will
become 1.
The Ladder Logic equivalent of a NOT gate is:
The AND Function
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The AND function is a very practical uses in PLC programming. The AND function will output the AND’d result
of two or more inputs on its input pins. Meaning, the output will be true only when both inputs are true. Let’s
look at this symbol little closer.
The truth table for the AND gate is:
The Ladder Logic equivalent of the AND gate is:
It is very clear from the truth table that the output will only be true (1) when both inputs A and B are true (1).
Please also take note how this is represented in Ladder Logic, it is read, “if A and B are true, then the output is
true”.
The OR Function
The OR function is another function that has very practical uses in PLC programming. The OR function will yield
a true (1) output when either A or B is true, or when both A and B are true. Let’s take a closer look at the OR
function.
The truth table for the OR gate is:
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The Ladder Logic equivalent of the OR gate is:
Looking at the truth table for the OR function it is very clear that when either A OR B are the output is also true.
In addition the output will be true if both A AND B are true as well. So in short, as long as at least one of the
inputs are true, the output will be true.
Also take not of how we represent a logical OR condition in the PLC using Ladder Logic. An OR condition is
created by “branching” the inputs around each other as illustrated.
PLC Basics – Combining AND/OR Functions with a NOT Function
The NAND Function
This is where things start to get interesting. If we combine the AND function with the NOT function we get
something called a NAND (NOT AND) function. I know, if things weren’t confusing enough right! As it turns the
NAND gate has an important role in the PLC world.
Let’s first look at the truth table to help clarify:
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Observing the truth table above we see that the output of the NAND function is essentially the inverted output of
the AND function. So long as A AND B are NOT true, the output is true!
Notice the little circle at the end of what is the AND gate. That little circle is what differentiates the NAND gate
from the AND gate letting the engineer know that this is NOT AND or NAND gate.
The Ladder Logic equivalent of the NAND gate is:
Pay particular attention to the Ladder Logic equivalent of the NAND. To build the NAND logic we must place the
A and B outputs in parallel (or branched) as we did for the OR. This time however, we are examining the NOT
state of each input.
Hopefully that’s clear!
The NOR Function
This time if we combine the NOT and the OR function we get something called…you guessed it, the NOR
function (NOT OR). Following the same reasoning as the NAND above, the output of the NOR function will yield
the inverted result of the OR function. Let’s take a look!
The truth table for the NOR function is:
Examining the truth table it is clear that the output for the NOR function is precisely the inverted output of the
OR.
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Similar to the NAND gate above, the little circle at the end of the OR is what tells us that it is NOT OR or the
NOR gate.
The Ladder Logic equivalent of the NOR gate is:
PLC Basics – Let’s Get Exclusive!
To complete our discussion of logic gates and boolean circuits there are two more functions we need to discuss.
These functions are the XOR (Exclusive OR) and the XNOR (Exclusive NOT OR).
The XOR Function
Believe it’s not as bad as at seems! The Exclusive OR, or XOR is one that we use almost every day in our decision
making process. Let me explain.
Imagine you are on a plane an the stewardess asks you if you would like a coffee or a tea. Let’s add one further
constraint to this in that you are only allowed one free beverage, so your choice is either coffee OR tea BUT
NOT both!
That is how the XOR works, if A is true (1) OR B is true (1) the output is true (1) so long as both A AND B are
NOT true (1). This is why we like truth tables…a picture says a 1000 words!
The truth table for the XOR function is:
As we said the output will be true (1), when either A OR B is true (1) but NOT when both A AND B are true (1).
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Notice the additional curved line added to the OR gate. This indicates that it is the Exclusive OR (XOR).
The Ladder Logic equivalent of the XOR gate is:
Spend some time to think through the logic and hopefully it will be clear! This bit of logic is read as follows, “If A
AND NOT B is true (1) then turn on the output”, OR, “If NOT A AND B is true (1), then turn on the output”.
Notice that only one of these statements can be true at any given time!
The XNOR Function
Last in our list of logic gates but certainly not least is the XNOR function (Exclusive NOT OR). If you’ve been
very studious thus far you can probably predict what the output of this function is going to be???
If you said the inverted output of the XOR function then you would be absolutely right! The XNOR is precisely
the inverted output of the XOR function. Let’s take a look!
The truth table for the XNOR function is:
Notice that it looks very similar to the XOR gate with the addition of the circular NOT symbol on the output.
The Ladder Logic equivalent of the XNOR gate is:
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PLC Basics – Programming Languages
PLC  programming involves “downloading” a compiled sequence of binary coded numbers into the PLC
system. There are various ways we can perform PLC programming tasks, however, we will focus on 3 of the most
common ways indicated by a red box in the image below.
A programming device is essential for writing, entering, and monitoring the PLC’s program. It communicates
directly with the  CPU and ensures the system functions correctly.
PLC  Programming Methods
We will focus on three in particular: Ladder Logic, Function Block, Structured Text or Statement Logic
PLC Basics – Introducing Ladder Logic Programming
The most common PLC Programming “language” is something referred to as Ladder Logic. Ladder Logic has
evolved from the days of controlling machines or processes using electro-mechanical relay devices or “relay
logic” as it is referred.
It is a graphical representation of the “relay contacts” that would have been used in a relay controlled system.
Each rung of ladder typically has one coil at the far right and then logical “input contacts” to the left.
-[    ]- Normally open (Examine if Closed) contact.
-[ ]- Normally closed (Examine if Opened) contact.
-(    )- Output Coil.
Here is a simple Start/Stop circuit with some compare instructions written in Ladder Logic.
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The input contacts are then arranged in a logical AND, OR type configuration to turn on an output as was
discussed in the previous section. We provide a comprehensive PLC instruction set list below so keep reading!
PLC Basics – Introducing Function Block Programming
Another common way to program a PLC is using Function Block. A Function Block Diagram (FBD) is a graphical
depiction of process flow using simple and complex interconnecting blocks. FBD’s are typically organized into
multiple “sheets” allowing one to organize the instruction sets – typically one sheet per device.
See below a sample function block routines for 4 different motor controllers.
It’s important to note that once the routine is executed all sheets in the list are executed as well. Below is a
Function Block program created using Studio 5000 software, making use of a PIDE (Proportional Integral
Derivative Enhanced) instruction to control a closed loop process. You can see that the instruction effectively get
“wired up” using the various input and output tags.
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PLC Basics – Introducing Structured Text Programming
Structured text is a textual programming language that uses statements to define what to execute. It follows more
traditional programming conventions that you would find with procedural type languages, however, typically it is
not case sensitive.
A series of statements (logic) is composed by formulating assignments and relationships using various operators
as depicted.
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In most cases where I’ve seen this type of PLC programming language used is when the programmer needs to
handle those types of functions or operations that can’t be defined clearly or efficiently using the other two
methods.
One word of caution – as a PLC programmer we always have to be cognoscente of who will be maintaining our
programs after the project is complete. In my experience, this often will be left to maintenance staff, namely,
electrical maintenance.
This is why ladder logic programming is by far the most common language used in industry today, because as we
mentioned early it evolves from the relay logic circuits of old that electricians in most facilities are familiar with.
PLC Basics – Common Instruction Sets
This is largely dependent on the PLC programming platform you are using, however, most PLC’s today will
include the following instruction sets or groups and this is not an exhaustive list by any means:
Bit Instructions – Binary type ON/OFF instructions XIC, XIO, OTL, OTU, OTE, ONS
Timer/Counter Instructions – TON, TOF, RTO, CTU, CTD, RES
Message/System Instructions – MSG, GSV, SSV
Compare Instructions – CMP, LIM, MEQ, EQU, LES, GRT, LEQ, GEQ
Math Instructions– CPT, ADD, SUB, MUL, DIV, MOD, SQR, NEG, ABS
Move/Logical Instructions – MOV, MVM, AND, OR, XOR, NOT, SWPF, CLR, BTD
File Manipulation Instructions – FAL, FSC, COP, FLL, AVE, SRT, STD, SIZE, CPS
File/Shift Instructions – BSL, BSR, FFL, FFU, LFL, LFU
Sequencer Instructions – SQI, SQO, SQL
Program Control Instructions – JMP, LBL, JSR, JXR, RET, SBR, TND, MCR, FOR, BRK
Motion Instructions – for PLC’s that support servo motion
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Advanced Math/Trig – for PLC’s that support advanced mathematical operations
Compact or integrated PLCs, also known as unitary PLCs, are designed for straightforward applications with a
fixed number of I/O points and an integrated  CPU. Unitary PLCs, also known as compact PLCs, are the
simplest type of PLC.
PLC Basics – Common PLC Acronyms
The following table shows a list of the some common acronyms or abbreviations you will see and hear when
researching  programmable logic controllers.
Edit
Acronym Description
ASCII American Standard Code for Information Interchange
BCD Binary Coded Decimal
CSA Canadian Standards Association
DCS Distributed Control System
DIO Distributed I/O
EIA Electronic Industries Association
EMI ElectroMagnetic Interferance
HMI Human Machine Interface
IEC International Electrotechnical Commission
IEEE Institute of Electrical and Electronic Engineers
I/O Input(s) and/or Output(s)
ISO International Standards Organization
LL Ladder Logic
LSB Least Significant Bit
MMI Man Machine Interface
MODICON Modular Digital Controller
MSB Most Significant Bit
NEC National Electrical Code
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PID Proportional Integral Derivative (feedback control)
RF Radio Frequency
RIO Remote I/O
RTU Remote Terminal Unit
SCADA Supervisory Control and Data Acquisition
TCP/IP Transmission Control Protocol/Internet Protocol
Conclusion
In conclusion, understanding the fundamentals of  Programmable Logic Controllers (PLCs) is essential for
anyone involved in industrial automation and control systems. PLCs have transformed the manufacturing
landscape by offering flexible, reliable, and cost-effective solutions for complex industrial processes.
From their basic architecture and components to advanced programming techniques like ladder logic and function
block diagrams, PLCs provide versatile tools for managing inputs and outputs, executing control logic, and
ensuring efficient operation of industrial systems.
As technology advances, the role of PLCs continues to expand, integrating with the Industrial Internet of Things
(IIoT) and other modern automation technologies. By mastering PLC basics, you can unlock the potential of
automation and drive innovation in your industrial applications.
Source: https://www.plcgurus.net/plc-basics/
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