PLC Basics and Advanced standard EGY.pptx

Programmable Logic Control
PLC Basics & Advanced Levels

What IS PLC
• Programmable Logic Controllers (PLCs) are
small industrial computers with modular
components designed to automate
customized control processes. PLCs are often
used in factories and industrial plants to
control motors, pumps, lights, fans, circuit
breakers and other machinery. To understand
the purpose of PLCs better, let’s look at a brief
history of PLCs.

History
• Industrial automation began long before PLCs. In
the early to mid 1900s, automation was usually
done using complicated electromechanical relay
circuits. However, the amount of relays, wires
and space needed to create even simple
automation was problematic. Thousands of
relays could be necessary to automate a simple
factory process! And if something in the logical
circuit needed to be changed?

History
• In 1968 the first programmable logic controller came
along to replace complicated relay circuitry in industrial
plants. The PLC was designed to be easily programmable
by plant engineers and technicians that were already
familiar with relay logic and control schematics. Since
the beginning PLCs have been programmable using
ladder logic which was designed to mimic control circuit
schematics. The ladder diagrams look like control
circuits where power is flowing from left to right
through closed contacts to energize a relay coil.

Ladder Logic Example
• As you can see, ladder logic looks like simple
control circuit schematics where input sources
like switches, push-buttons, proximity sensors,
etc are shown on the left and output sources
are shown on the right. The ability to program
complicated automated processes with an
intuitive interface like ladder logic made the
transition from relay logic to PLCs much
simpler for many in the industry.

How Do PLCs Work?
• PLCs can be described as small industrial computers
with modular components designed to automate
control processes. PLCs are the controllers behind
almost all modern industrial automation. There are
many components to a PLC, but most of them can
be put in the following three categories:
• Processor (CPU)
• Inputs
• Outputs

How Do PLCs Work?
• PLCs are complex and powerful computers. But, we can
describe the function of a PLC in simple terms. The PLC
takes inputs, performs logic on the inputs in the CPU and
then turns on or off outputs based on that logic. We will
get into more detail later but for now, think of it like this:
• The CPU monitors the status of the inputs (ex. switch on,
proximity sensor off, valve 40% open, etc.)
• The CPU takes the information that it gets from the inputs,
performs logic on the inputs
• The CPU operates the outputs logic (ex. turn off motor,
open valve, etc.)

PLC Function Flowchart
• Let’s use a familiar example to illustrate how PLCs work. Your
dishwasher. Many dishwashers have microprocessors that function
similarly to PLCs. The dishwasher has inputs, outputs and, of course,
a CPU. Some of the inputs into the dishwasher controller would be
the buttons on the front, the water sensors and the door switch.
Some of the dishwasher outputs would be the water valves, the heat
elements and the pumps. Now let’s think about how the dishwasher
uses those different components.
NOTE: Remember, the CPU is the processor in the dishwasher that is
programmed to make all the decisions we will see below. This is just
like a PLC processor (CPU) which makes logical decisions based on
input status.

• User pushes the cycle mode button (input
detected)
• User pushes the start button (input detected)
• CPU verifies that the door is closed (input
detected)
• Fill valve opens and the dishwasher begins filling
with water (output activated)
• CPU waits until proper water level is reached
(input detected)

• Fill valve closes, and water flow stops (output
activated/de-activated)
• Heating element is turned on (output activated)
• CPU waits until proper water temperature is
reached (input detected)
• Soap dispenser opens (output activated)
• Water pump turns on to force water through
sprayers (output activated)

• CPU begins timing depending on cycle type (logic
timer activated)
• Water pump turns off (output deactivated)
• Heating element is turned off (output
deactivated)
• Drain valve opens and the dishwasher begins
draining the dirty water (output activated)
• CPU waits until it detects the water level to be
low enough (input activated/de-activated)

• Drain valve closes (output
activated/deactivated)
• Fill valve opens again to rinse dishes (output
activated)
• Water pump turns on to force water through
sprayers (output activated)
• CPU begins timing (logic timer activated)
• Water pump turns off (output deactivated)

• Drain valve opens and the dishwasher begins
draining rinse water (output activated)
• CPU waits until it detects the water level to be low
enough (input activated/de-activated)
• Drain valve closes (output activated/deactivated)
• Heating element turns on to heat the air inside the
dishwasher and dry the dishes (output activated)
• CPU waits until proper interior temperature is
reached (input activated)

• CPU begins timing (logic timer activated)
• Heating element is turned off (output
activated/deactivated)

Discrete and Analog I/O
• Inputs and outputs are often abbreviated with the term
“I/O”. In the dishwasher example above, we treated every
input and output as a discrete or digital signal. Discrete
signals are signals that can only be on or off. These are the
simplest and most common type of I/O. In our example we
did not use any analog I/O. Although, there may be some use
of analog I/O within a dishwasher control system, I wanted to
keep this example simple. With analog signals, instead of
only on/off or open/closed possibilities, you may have 0 –
100%, 4 – 20mA, 0 – 100 degrees Celsius, or whatever it is
you measuring as an input or driving as an output.

Discrete and Analog I/O
• You may have heard of the Programmable Automation Controller
(PAC). The term was first coined by the market research firm ARC in
2001 to differentiate the original PLCs from the newer, more
powerful, more flexible controllers that were entering the market.
There is disagreement about the definition differences between
PAC and PLC, and often the terms are used interchangeably in the
industry. I often use the terms interchangeably myself. This article,
here, from Control Engineering may help you understand the
differences between PLCs and PACs. In my opinion PACs are always
the better choice unless the system is very simple and minimizing
cost of the project is vital. The modern user interface, extra power
and memory of most PACs make them easily superior to most PLCs.

The Need for PLCs
• Hardwired panels were very time consuming to
wire, debug and change.
• GM identified the following requirements for
computer controllers to replace hardwired panels.
• Solid-state not mechanical
• Easy to modify input and output devices
• Easily programmed and maintained by plant
electricians
• Be able to function in an industrial environment

The First Programmable Logic Controllers
(PLCs)
• Introduced in the late 1960’s
• Developed to offer the same functionality as the
existing relay logic systems
• Programmable, reusable and reliable
• Could withstand a harsh industrial environment
• They had no hard drive, they had battery backup
• Could start in seconds
• Used Ladder Logic for programming

Programmable Logic Controller
• A programmable logic controller (PLC) is a
specialized computer used to control
machines and process.
• It uses a programmable memory to store
instructions and specific functions that include
On/Off control, timing, counting, sequencing,
arithmetic, and data handling

Advantages of PLC Control Systems
• Flexible
• Faster response time
• Less and simpler wiring
• Solid-state - no moving parts
• Modular design - easy to repair and expand
• Handles much more complicated systems
• Sophisticated instruction sets available
• Allows for diagnostics “easy to troubleshoot”
• Less expensive

Advantages of a PLC Control System
• Eliminates much of the hard wiring that was
associated with conventional relay control
circuits.
• The program takes the place of much of the
external wiring that would be required for
control of a process.

• Increased Reliability: Once a program has
been written and tested it can be downloaded
to other PLCs.
• Since all the logic is contained in the PLC’s
memory, there is no chance of making a logic
wiring error. Conversely ......

• More Flexibility: Original equipment
manufacturers (OEMs) can provide system
updates for a process by simply sending out a
new program.
• It is easier to create and change a program in a
PLC than to wire and rewire a circuit. End-
users can modify the program in the field.

• Lower Costs: Originally PLCs were designed to
replace relay control logic. The cost savings
using PLCs have been so significant that relay
control is becoming obsolete, except for
power applications.
• Generally, if an application requires more than
about 6 control relays, it will usually be less
expensive to install a PLC.

• Communications Capability: A PLC can
communicate with other controllers or
computer equipment.
• They can be networked to perform such
functions as: supervisory control, data
gathering, monitoring devices and process
parameters, and downloading and uploading
of programs.

• Faster Response Time: PLCs operate in real-
time which means that an event taking place
in the field will result in an operation or
output taking place.
• Machines that process thousands of items per
second and objects that spend only a fraction
of a second in front of a sensor require the
PLC’s quick response capability.

• Easier To Troubleshoot: PLCs have resident
diagnostic and override functions allowing
users to easily trace and correct software and
hardware problems
• The control program can be watched in real-
time as it executes to find and fix problems

PLC Architecture
• An open architecture design allows the system to be
connected easily to devices and programs made by
other manufacturers.
• A closed architecture or proprietary system, is one
whose design makes it more difficult to connect
devices and programs made by other manufacturers.
• NOTE: When working with PLC systems that are
proprietary in nature you must be sure that any
generic hardware or software you use is compatible
with your particular PLC.

I/O Configurations
• Fixed I/O
• Is typical of small PLCs
• Comes in one package, with no separate
removable units.
• The processor and I/O are packaged together.
• Lower in cost – but lacks flexibility.

I/O Configurations
• Modular I/O
• Is divided by compartments into which
separate modules can be plugged.
• This feature greatly increases your options and
the unit’s flexibility.
• You can choose from all the modules available
and mix them in any way you desire.

I/O Configurations
• Modular I/O
• When a module slides into the rack, it makes
an electrical connection with a series of
contacts - called the backplane. The backplane
is located at the rear of the rack.

Power Supply
• Supplies DC power to other modules that plug
into the rack.
• In large PLC systems, this power supply does
not normally supply power to the field
devices.
• In small and micro PLC systems, the power
supply is also used to power field devices.

Processor (CPU)
• Is the “brain” of the PLC.
• Consists of a microprocessor for implementing the
logic, and controlling the communications among the
modules.
• Designed so the desired circuit can be entered in relay
ladder logic form.
• The processor accepts input data from various sensing
devices, executes the stored user program, and sends
appropriate output commands to control devices.

I/O Section
• Consists of:
• Input modules
• Output modules.

I/O Section
• Input Module
• Forms the interface by which input field
devices are connected to the controller.
• The terms “field” and “real world "are used to
distinguish actual external devices that exist
and must be physically wired into the system.

I/O Section
• Output Module
• Forms the interface by which output field
devices are connected to the controller.
• PLCs employ an optical isolator which uses
light to electrically isolate the internal
components from the input and output
terminals.

Programming Device
• A personal computer (PC) is the most
commonly used programming device
• The software allows users to create, edit,
document, store and troubleshoot programs
• The personal computer communicates with
the PLC processor via a serial or parallel data
communications link

Programming Device
• Hand-held programming devices are sometimes
used to program small PLCs
• They are compact, inexpensive, and easy to use,
but are not able to display as much logic on
screen as a computer monitor
• Hand-held units are often used on the factory
floor for troubleshooting, modifying programs,
and transferring programs to multiple
machines.

PLC Mixer Process Control Problem
• Mixer motor to automatically stir the liquid in
the vat when the temperature and pressure
reach preset values
• Alternate manual pushbutton control of the
motor to be provided
• The temperature and pressure sensor
switches close their respective contacts when
conditions reach their preset values

PLC Mixer Process Control Problem

Process Control Relay Ladder Diagram
• Motor starter coil is energized when both the
pressure and temperature switches are closed
or when the manual pushbutton is pressed

PLC Input Module Connections
• The same input field devices are used
• These devices are wired to the input module
according to the manufacturer’s labeling
scheme

PLC Output Module Connections
• Same output field device is used and wired to
the output module
• TRIACswitches motor ON and OFF in
accordance with the control signal from the
processor

PLC Ladder Logic Program
• The format used is similar to that of the hard-
wired relay circuit

PLC Ladder Logic Program
• I/O address format will differ, depending on
the PLC manufacturer. You give each input and
output device an address. This lets the PLC
know where they are physically connected

Entering And Running The PLC Program

PLC Operating Cycle
• During each operating cycle, the controller
examines the status of input devices, executes
the user program, and changes outputs
accordingly
• The completion of one cycle of this sequence
is called a scan. The scan time, the time
required for one full cycle, provides a measure
of the speed of response of the PLC

Modifying A PLC Program
• The change requires that the manual
pushbutton control should be permitted to
operate at any pressure but not unless the
specified temperature setting has been
reached.

• If a relay system were used, it would require
some rewiring of the system, as shown, to
achieve the desired change.

• If a PLC is used, no rewiring is necessary! The
inputs and outputs are still the same. All that
is required is to change the PLC program

PLCs Versus Personal Computers

PC Based Control Systems
• Advantages
• Lower initial cost
• Less proprietary hardware and software
required
• Straightforward data exchange with other
systems
• Speedy information processing - Easy
customization

PLC Size Classification
• Criteria
• Number of inputs and outputs (I/O count)
• Cost
• Physical size

IEC 61131-3
• The IEC (International Electro technical
Commission) is a worldwide organization for
standardization comprising all national electro
technical committees (IEC National
Committees)
• This part of IEC 61131 specifies syntax and
semantics of programming languages for
programmable controllers as defined in part 1
of IEC 61131.

IEC 61131-3
• This part of IEC 61131 specifies the syntax and semantics of a
unified suite of programming languages for programmable
controllers (PCs).
• These consist of two textual languages, IL (Instruction List) and
ST (Structured Text), and two graphical languages, LD (Ladder
Diagram) and FBD (Function Block Diagram).
• Sequential Function Chart (SFC) elements are defined for
structuring the internal organization of programmable
controller programs and function blocks. Also, configuration
elements are defined which support the installation of
programmable controller programs into programmable
controller systems.

1.4.3 Programming model
• The elements of programmable controller programming languages, and the sub
clauses in which they appear in this part, are classified as follows:
• Data types (2.3)
• Variables (2.4)
• Program organization units (2.5)
• Functions (2.5.1)
• Function blocks (2.5.2)
• Programs (2.5.3)
• Sequential Function Chart (SFC) elements (2.6)
• Configuration elements (2.7)
• Global variables (2.7.1)
• Resources (2.7.1)
• Access paths (2.7.1)
• Tasks (2.7.2)

1.4.3 Programming model
• Combination of programmable controller
language elements
• LD - Ladder Diagram (4.2)
• FBD - Function Block Diagram (4.3)
• IL - Instruction List (3.2)
• ST - Structured Text (3.3) OTHERS –
• Other programming languages (1.4.3)

2.2.2 Character string literals

Program Organization Unit(POU)
• This is an object that holds logic that is used to
develop your application. These can be declared as
various different types (which changes their
behavior), but POUs ultimately serve one function
—to hold and execute your code. As well as being
declared as different types (which we'll come on
to), POUs can also be declared as using a different
language. This doesn't mean a different spoken
language like English, but a different programming
language

Task
• A Task is exactly what it sounds like; it's a Task
that tells your application to run a set of POUs
or gather IO data. In some PLCs, Tasks perform
various other tasks too and may not be called
"Tasks" at all (looking at you Siemens, OB1,
OB35, etc. are basically Tasks).
• In most PLCs, Tasks can be defined with a
range of various parameters such as

Task
• Task Mode: The mode the task is operating in, such
as Cyclic Execution, Event Driven, Freewheeling. It's
probably best to look up the different modes available
and what they mean to the PLC you are using, as they're
not always done in the same way.
• Watchdog Timeout: The time in which the entire
task MUST complete. Failing to complete the task in this
time will raise an internal flag that drops all outputs to a
safe state. Some PLCs allow you to configure what
happens on Watchdog failure; some don't. Refer to the
documentation for your own PLC.

Task
• An important rule to remember is that if a POU cannot be
traced back to a Task, it will not be executed. For example:
• Task >> Main (PRG) >> Sub (PRG) >> Area_1 (FB) >>
Function (FB)
• The above shows "Task" calling "Main" which is calling
"Sub" and so on. If "Area_1" was deleted, "Function"
would have no route to a Task and would therefore no
longer be executed in the program. Most (not all) PLC
programming environments tell you that a POU is
orphaned from a Task.

PROGRAM
• A PRG is a type of POU in most PLCs (Not all, again looking at
Siemens in which PRG doesn't exist). At least one PRG must exist
as Tasks can only call a PRG. Because a PRG is simply a type of
POU, it performs in the same manner as any other POU and can
be declared in different languages.
• A PRG can call another PRG as well as call any other type of POU.
A PRG can also declare its own Variables (Covered later).
• Note: In some PLCs, PRGs can declare their own variables, but
they are not maintained between PLC scans (a complete
execution of a task); this means that any value written to the
variable is lost at the end of the scan. These types of variables
are usually referred to as Temp Variables.

Function Block
• A Function Block is probably the most common
POU used in a PLC. They are used to create
blocks of code that can be used over and over
again by simply dropping the FB into a POU or
another FB. FBs are made up of Input and
Output parameters (we'll cover these in more
detail) that allow data from outside the FB to
be brought in and data made by the FB to be
passed back out to the caller. For example

Function Block
• The above shows FB_1 being called on line 1 (a PRG is
calling it). The input data has Sensor_1 being passed to it.
The FB_1 object is performing a task and then
outputting Output, which is being passed to Output in the
PRG that is calling the FB.
• Line 2 is showing FB_1_CALL.Counter being used, but we
cannot see "Counter" as a parameter of FB_1? This is
because "Counter" is a Static Variable (A variable that is
used to hold information rather than pass it anywhere). In
most PLCs, Static Variable information is accessible if
the Instance of that data is also declared

What is Instance Data?
• Instance data is the data that belongs to an FB. In
the example above, FB_1_CALL holds all instance
data of FB_1. This is why declaring
"FB_1_CALL.Counter" works correctly. FB_1 is the
name of the FB, FB_1_CALL is the data for that
specific call of that FB.
• If FB_1 was called again on Line 3, you would need
to give it a different set of instance data by declaring
a different identifier for it, such as "FB_1_CALL2".

FUNCTION
• A function is very similar to a Function Block, but it does
not hold its own data for more than 1 PLC scan; all
variables are temporary
• PLCs handle functions in different ways; for example
CoDeSys allows you to leave interface pins unassigned
whereas Siemens does not. Most PLCs also enforce that a
variable is returned when the Function completes. This
variable must be declared when the Function is created.
It's very common to see functions returning
a Byte or Word which contains a status on whether the
Function completed without issue.

VARIABLE
• A Variable is a container that holds information; there are many different types,
and it depends on the PLC in use. The main Variable types (also known as Data
Types) are:
• BOOL: Digital Data (True / False)
• BYTE: Numerical Data / Bitwise Data (0 - 255)
• INT: Numerical Data (-32768 - 32767)
• UINT: Numerical Data (0 - 65535)
• SINT: Numerical Data (-128 - 127)
• USINT: Numerical Data (0 - 255)
• DINT: Numerical Data (-2147483648 - 2147483647)
• WORD: Numerical Data / Bitwise Data (0 - 65535)
• DWORD: Numerical Data / Bitwise Data (0 - 4294967295)
• REAL: Numerical Data (-3.402823e+38 - 3.402823e+38)
• ARRAY: Array of Any Data type (Declared as "ARRAY [0..10] OF Data Type)

VARIABLE
• Most PLCs support the above, and some PLCs will
support a selection of the below also:
• LWORD: Numerical Data / Bitwise Data (0 -
18446744073709551615)
• UDINT: Numerical Data (0 - 4294967295)
• LINT: Numerical Data ( -9,223,372,036,854,775,808 -
9,223,372,036,854,775,807)
• ULINT: Numerical Data (0 - 18446744073709551615)
• VARIANT: Object (Anything)
• NULL: Object (Nothing)

VARIABLE
• The additional variables are generally only supported by 64bit PLCs and
Runtimes. Variant & Null data types are advanced and not common in PLCs.
• In addition to the above Data Types, there are also different Variable
attributes (modes if you like):
• CONSTANT - Variable that is hard coded and cannot be changed at runtime
• RETAIN - Variable that remembers its last value between loss of power supply
to the PLC. Most PLCs have a limit on the maximum amount of data that can
be retained. Older PLCs may retain everything by default or have special
ranges of registers that are retained, so make sure you check.
• PERSISTENT - A variable that retains it's last value even after a re-initialization
of the PLC or the PLC is warm started. The only way to reload the default data
is to cold start the PLC or perform a full download.
• Note: Persistent variables can be dangerous if used incorrectly, especially if
indirect addressing / pointers are being used.

INTERFACE
• An interface is the declaration of variables a PRG, FB or FC is expecting to use.
There are a few keywords that can be used to declare interfaces:
• VAR_INPUT - Data passed into the POU
• VAR_OUTPUT - Data passed out of the POU
• VAR_IN_OUT - Data that is passed in and out of the POU to the same variable
(If you know a bit about computer programming, think of this as passing by
reference)
• VAR - Data that is local to the POU, Some PLCs allow access to the data by
explicit reference only (For example, "POU.VARIABLE")
• VAR_STATIC - The same as VAR, but does not allow access to the data from
outside the block
• VAR_TEMP - Temporary data, the values stored in TEMPs is lost when the block
is exited
• END_VAR - A required termination declaration after declaring your variables.

VAR_GLOBAL
• GLOBAL Variables are special variables that are accessible
anywhere in a project. They serve as a great way of passing
information between different areas of your project.
• Some people use Globals for everything and don't declare
any VARs in POUs. I advise against this as it gets messy
quickly!
• Globals are usually defined in a special Global Variable list
or Symbol table, depending on the PLC you are using.
• (Siemens use DBs, variables stored in DBs that are not
Instance DBs are the equivalent of Global Variables)

POU Languages
• LADDER
• Ladder is probably the most commonly used
language. It's easy to read and follow and fault
find

POU Languages
• FUNCTION BLOCK DIAGRAM
• FBD is very, very similar to Ladder; it tends to
be used for projects that are made up of many
separate functions (hence the name). Logic
that compares Bool values is easier in Ladder
than it is in FBD.

POU Languages
• STRUCTURED TEXT
• Structured Text is one of (if not, the most)
flexible of the languages. It's quick to program
in and easy to read but can get messy quickly
if formatting rules aren't followed.

POU Languages
• Sequential Function Chart
• This language is excellent for sequencing (hence
the name!). However, it is one of the more
difficult to understand. In the example below, it
is important to note that the "ProcessTimer"
step must be called in any scenario; else, the
timer will not update and will hold its last value.
It is very easy to get stuck with SFC and leave
variables in states that were not intended.

POU Languages
• CONTINUOUS FUNCTION CHART
• CFC is very similar to FBD, but you are not confined
to networks (horizontal placeholders); you are free
to draw your logic however you like. This language is
useful for electricians transferring to PLC logic, as it
reads the same as a drawing. There are a few things
to be careful of, though; the logic may not flow as
expected. There are small numbers that show the
logic flow, it's important to keep track of what is
happening and where.

LIBRARIES
• Libraries are a collection of POUs and Variable
lists that can be moved from project to project.
This allows you to have a standard set of POUs,
tried and tested, that can be dropped into a
project when required.
• Libraries can be nested, too, so a library can
call another library if required. Any large-scale
software house will almost definitely have a
standard library set.

What is Tag?
• Tag is a name you assign to an address of
device/PLC.
• It is also called "variable" or "symbol"
depending on the manufacture of the
device/PLC.

Ladder Logic Basics
• Ladder logic is a programming language that is used to
program a PLC (Programmable Logic Controller). It is a
graphical PLC programming language which expresses logic
operations with symbolic notation using ladder diagrams,
much like the rails and rungs of a traditional relay logic circuit.
• Ladder logic is a fast and simple way of creating logic
expressions for a PLC in order to automate repetitive machine
tasks and sequences. It is used in a multitude of industrial
automation applications. Some industrial automation
application examples where PLC ladder logic is used include….

Ladder Logic Basics
• Material Handling Conveyor System.
• Pallet Packing and Strapping.
• Ball Mill Lubrication System.
• Logistics Package Conveying and Sorting.
• Cement Batching.
• Beverage Bottling and Labeling.
• Hopper and Tank Level Control.
• Air and Liquid Flow and Pressure Control.
• In the good ol’ days, machine and process automation was accomplished
using a hard wired control system known as relay logic. With the advent
of microprocessors and the invention of the PLC, relay logic quickly
became superseded by programming languages such as ladder logic.

Why is Ladder Logic Popular?
• Ladder logic is the most popular method of PLC programming
because it has an easy to use graphics based interface and the
programming language resembles an electrical schematic
drawing. Engineers, electricians and students find the
transition from an electric circuit to ladder logic relatively
easy.
• When programming ladder logic in a PLC, the graphic, drag and
drop nature of ladder diagrams helps you formulate code
quickly and easily. Ladder logic also helps you easily trouble
shoot your code because you can visually see the flow of logic
from the LHS start rail, through the logic symbols and to the
RHS end rail.

Learning the Basics of Ladder Logic
• It’s relatively easy to learn the basic concepts of ladder logic
programming, even if you don’t have experience with electric circuits.
Take comfort in knowing that ladder logic is the quickest and easiest PLC
programming language to learn.
• In order to help you learn the basics of ladder logic we will cover the
following….
• Introduce the ladder diagram.
• Examine the seven basic parts of a ladder diagram.
• Identify the binary and logic concepts used in ladder logic.
• Reveal the hidden ladder logic functions that are automatically built
into the structure of the ladder diagram.
• Discover the five fundamental logic functions that are essential to
know.

What is a Ladder Diagram in a PLC?
• A ladder diagram is the symbolic representation of the control
logic used for programming of a PLC. Ladder diagrams have
horizontal lines of control logic called rungs and vertical lines at
the start and end of each rung called rails. It looks just like a
ladder, hence the name “ladder diagram”.
• There are two main differences between an electrical
schematic and a ladder diagram:
• The control logic in an electrical schematic is represented using
components whereas in a ladder diagram symbols are used.
• The control logic execution in an electrical schematic is as per
the operation of an electrical circuit whereas in a ladder diagram
it relies on the methodical nature of the PLC scan

Why is a ladder diagram used for PLC
programming?
• Ladder diagrams are used to formulate PLC logic
expressions in graphical form. They use symbols to
represent conditional, input and output expressions.
Ladder diagrams are similar to relay control circuits and
are used due to their ease of programming compared to
text based programming languages.
• Early control system designers were accustomed to relay
logic control circuits and ladder diagrams closely mimic
these. They preferred to use ladder diagrams for PLC
programming instead using text based programming
languages of the day like C, BASIC, Pascal and FORTRAN.

Why is a ladder diagram used for PLC
programming?
• Factory maintenance staff already understand how to read relay control
circuits. They can use their knowledge of relay control circuits to help
troubleshooting control system problems that implement PLC
programming with ladder diagrams.
• Ladder Diagram (LD) is the official name given in the international PLC
programming standard IEC-61131. But, these days the terms ladder
diagram, ladder logic diagram, ladder drawing, ladder control, ladder
circuit, control logic diagram and logic diagram (to name a few) are all
used to describe relay logic circuits and ladder logic programming.
• So don’t get too caught up in the specific definition of each of these
expressions, they kind of generally all mean the same thing. At the end of
the day most people will know what you are talking about anyway.
Personally, I use the term ladder logic for PLC programming and relay
logic for relay control circuits.

How to Draw Ladder Logic Diagrams?
• Ladder logic diagrams are drawn in a similar way to relay logic
circuit. They use rails and rungs to create the logic framework.
The logic operations are drawn in using symbolic notation.
• The rails in a relay logic circuit represent the supply wires of a
relay logic control circuit. However, in ladder diagrams, the rails
represent the start and end of each line of symbolic code.
• The rungs in a relay logic circuit represent the wires that
connect the components together. However, in a ladder
diagrams, the rungs represent the logic flow through the
symbolic code.

• When implementing a ladder logic program in a PLC there are
seven basic parts of a ladder diagram that critical to know. They
are rails, rungs, inputs, outputs, logic expressions, address
notation/tag names and comments. Some of these elements
are essential and others are optional.
• To help understand how to draw ladder logic diagrams the
seven basic parts of a ladder diagram are detailed below…..
• Rails – There are two rails in a ladder diagram which are drawn
as vertical lines running down the far most ends of the page. If
they were in a relay logic circuit they would represent the active
and zero volt connections of the power supply where the power
flow goes from the left hand side to the right hand side.

• Rungs – The rungs are drawn as horizontal lines and connect the rails
to the logic expressions. If they were in a relay logic circuit they would
represent the wires that connect the power supply to the switching
and relay components. Each rung is numbered in ascending sequential
order.
• Inputs – The inputs are external control actions such as a push button
being pressed or a limit switch being triggered. The inputs are actually
hardwired to the PLC terminals and represented in the ladder diagram
by a normally open (NO) or normally closed (NC) contact symbol.
• Outputs – The outputs are external devices that are being turned on
and off, such as an electric motor or a solenoid valve. The outputs are
also hardwired to the PLC terminals and are represented in the ladder
diagram by a relay coil symbol.

• Logic Expressions – The logic expressions are used in combination
with the inputs and outputs to formulate the desired control
operations.
• Address Notation & Tag Names – The address notation describes the
input, output and logic expression memory addressing structure of
the PLC. The tag names are the descriptions allocated to the
addresses.
• Comments – Last but by not least, the comments are an extremely
important part of a ladder diagram. Comments are displayed at the
start of each rung and are used to describe the logical expressions
and control operations being executed in that rung, or groups of
rungs. Understanding ladder diagrams is made a lot easier by using
comments.

How Does Ladder Logic Work?
• Ladder logic works in a similar way to relay logic, but without
all the laborious relay control wiring. In simple terms, the field
input and output devices are wired directly to the PLC and the
ladder logic program decides what outputs to activate,
depending on the status of the input signals.
• Just like relay logic, ladder logic has supply rails, relay coils, relay
contacts, counters, timers, PID loop controllers and much more.
The difference is that with relay logic the logic expressions are
created with relay control circuits. This can amount to large
amounts of relays and wiring. However, with ladder logic the
logic expressions are programmed in the PLC. So, the only
wiring required is for the input and output devices.

How to Read Ladder Logic?
• Ladder logic is read from the left hand rail to the right
hand rail and from the first rung to the last rung. In
short – LEFT TO RIGHT AND TOP TO BOTTOM. The
rungs contain input symbols that either pass or block
the logic flow. The result of the rung is expressed in the
last symbol, known as the output.
• To start reading ladder logic we need to know some
basic binary concepts, how they apply to ladder
logic, how ladder logic is executed and the basic logic
functions that are built into each rung. Let’s begin….

The Binary Concept Applied to Ladder Logic
• Microprocessors, like the ones found in PLCs and personal
computers operate on the binary concept. You’ve probably heard
of the term ‘binary’. It refers to the principle that things can be
thought of in one of two states. The states can be defined as:
• True or False
• 1 or 0
• On or Off
• High or Low
• Yes or No
• Microprocessors love binary…..
10101011101000111010001010100010100100101010010011.

• I don’t know about you, but my head hurts just looking at that!
Luckily ladder logic uses symbolic expressions and a graphical
editor for writing and reading ladder diagrams making it easier for
us mere humans to comprehend.
• In a PLC, binary events are expressed symbolically using ladder
logic in the form of a normally open contact (NO) and normally
closed contact (NC).
• The normally open contact (NO) is TRUE when the event is active
and FALSE when the event is NOT active. While the normally
closed contact (NC) is FALSE when the event is active and TRUE
when the event is NOT active.
• Let me explain NO and NC contacts a little further …..

• Normally Open Contact (NO) in Ladder Logic
• The event associated with a normally open contact (NO) can be
TRUE or FALSE. When the event is TRUE then it is highlighted green
and the logic flow can move past it to the next logic expression. Just
like the current flow in an electric circuit when a switch is turned on.
• Let’s call a certain PLC input event ‘A’. This PLC input event could be
something like a button being pushed, a limit switch being activated
or a temperature switch being triggered.
• PLC input event ‘A’ follows the binary concept and has one of two
states, TRUE or FALSE. The ladder logic truth table for a normally
open contact (NO) which denotes PLC input event ‘A’ is shown
below….

• Normally Closed Contact (NC) in Ladder Logic
• The event associated with a normally closed contact
(NC) can be TRUE or FALSE. The result of the
normally closed contact (NC) is basically the opposite
state of an event that occurs. So, if PLC input A is
FALSE the result will be TRUE. And vise versa when
PLC input A is TRUE the result will be FALSE.
• The normally closed contact (NC) is considered to be
a ladder logic NOT function. It is sometimes referred
to as reverse logic. Check out the truth table below….

• If we translate a NOT function into a ladder
logic diagram we express it symbolically in the
form of a normally closed contact (NC) as
seen in ladder logic truth table shown
below….

How Ladder Logic is Executed?
• In order to successfully read ladder logic we need a basic understanding of
how a PLC works and how ladder logic is executed in a PLC. You see, the PLC
follows a certain execution procedure and if not adhered to it can lead to the
ladder logic being read incorrectly.
• Ladder logic works in a similar way to relay logic, but without all the
laborious wiring. It has supply rails, relay coils, relay contacts, counters,
timers, PID loop controllers and much more. In simple terms, all the field
input and output devices are wired to the PLC and the ladder logic program
decides what outputs to trigger depending on the status of the input signals.
• In basic terms, PLCs execute ladder logic by first reading all the input states
and storing them into memory. Secondly, scanning through and evaluating
each rung of ladder logic, from left to right and top to bottom. Lastly, at the
end of the scan, the resultant logic is executed and the outputs are written
to.

Ladder Logic Basic Functions
• In a ladder diagram the normally open (NO) and normal closed (NC)
contacts merely tell us what state an event is in, TRUE or FALSE. On
their own they cannot decide what action to take to automate
something.
• We need binary’s best friend ‘logic’ to help out.
• Logic is the ability to decide what action needs to be taken depending
on the state of one or more events. We use the binary and logic
concepts every day in our own lives. For example, if I feel cold then I
put my sweater on, but if I feel hot then I take my sweater off.
• Binary concept – Cold or Hot, Sweater On or Sweater Off.
• Logic concept – IF, THEN logic functions.
• Binary Logic in action!

• The binary and logic concepts are what makes ladder logic
work. The hidden key to unlock your understanding of how
ladder logic works is: The logic functions in ladder logic are
automatically built into the structure of the ladder diagram.
• Let me show you……
• Ladder Logic IF, THEN Functions
• Let’s take a real world event, allocate it to a normally open
contact (NO) and call it ‘A’. In ladder logic the real world events
are defined as PLC inputs.
• Now, let’s call the result of the logic function ‘Y’. In ladder logic
the result of a rung logic function is defined as a PLC output.

• When we take these two fundamental
elements and insert them into a rung in a
ladder diagram we get your first line of code!
• It’s equivalent to “Hello World” in text based
programming languages…..

• Now, let’s expose the hidden inbuilt functions
by highlighting them in blue in order to
illustrate the relationship between the ladder
diagram rung structure and its inbuilt IF, THEN
functions….

• We can write out the logic expression in the
above as rung as IF A THEN Y.
• Because PLC input A follows the binary concept it
has two possible states, TRUE or FALSE. Therefore
it results in two possible logic iterations:
• IF A = FALSE THEN Y = FALSE
• IF A = TRUE THEN Y = TRUE
• We also can express this in a truth table….

• If we translate this into a ladder logic diagram we express it
symbolically in the form of a normally open contact (NO) for
the input and a relay coil for the output. Remember the logic
flow is from left to right and follows the same concept of
current flow in an electric circuit.
• The ladder logic truth table is shown below….

• The AND function examines multiple PLC inputs and has one resulting
output. If we translate an AND function into a ladder diagram we can
express it symbolically in the form of two PLC inputs A and B using
normally open (NO) contacts and a PLC output Y using a relay coil.
• They are all connected in line, just like a series connection in an electric
circuit. This time we have also highlighted the hidden AND function to
illustrate the relationship between the ladder logic functions and the
ladder diagram rung structure….

• We can write out the logic expression above as IF A AND B THEN Y.
• The AND function examines if all the PLC inputs are TRUE, then the
corresponding result is also TRUE. However if any one of the PLC inputs is
FALSE then the corresponding result is also FALSE.
• Because PLC input A and B follows the binary concept and are part of the
AND function there are four possible logic iterations. Check out the truth
table below….

• The number of logic iterations increases with the number of PLC inputs
(2PLC Inputs
). But that doesn’t matter too much with the AND function
because the result can only be TRUE if all the PLC inputs are TRUE.
• If we translate an AND function into a ladder logic truth table we get the
table below….

• Ladder Logic OR Function
• The OR function examines multiple PLC inputs and has one resulting
output. If we translate an OR function into a ladder diagram we can
express it symbolically in the form of two PLC inputs A and B using
normally open contacts (NO) and a PLC output Y using relay coil.
• The inputs are placed in the rung in what is known as a branch. This is the
equivalent of a parallel connection in an electric circuit. The output is then
connected in line with the rung. This time we have also highlighted the
hidden OR function when we create a branch (parallel connection) with
PLC input B across PLC input A….

• We can write out the logic expression above as IF A OR B THEN Y.
• The OR function examines if any of the PLC inputs are TRUE, then the
corresponding result is also TRUE. However, all the PLC inputs must be
FALSE in order for the corresponding result is also be FALSE.
• Because PLC input A and B follows the binary concept and are part of the
OR function there are four possible logic iterations. Check out the truth
table below….

• Remember, the number of logic iterations increases with the number of
PLC inputs (2PLC_inputs
). But that doesn’t matter too much with the OR
function because the result can be TRUE if any of the PLC inputs are TRUE.
• If we translate an OR function into a ladder logic truth table we get the
table below….

• Wow, you’ve flown through the binary and logic functions. Remember…
• For basic ladder logic programming we express binary events using
normally open contacts (NC) and normally closed contacts (NC).
• The five basic, yet essential, logic functions in ladder logic are:
• NOT
• IF
• THEN
• AND
• OR

What is a Functional Block Diagram?
• A Functional Block Diagram (abbreviated as FBD) is a
graphical representation of a functional process via blocks
and diagrams that is easier for a reader to understand and
interpret. An FBD helps us determine the function
between output variables and input variables via a set of
rudimentary blocks and diagrams that are connected with
arrows known as “connections.”
• A Functional Block Diagram can help us create
relationships between one or more than one variable
(both input and output) to establish our understanding of
functional processes aligned in a system.

Where are Functional Block Diagrams Being
Used?
• These diagrams help us understand the
functions and relationships between two or
more variables widely used in software
engineering, system engineering, and graphical
programming language. For software engineers
and programmers, FBD is an essential tool that
helps them understand and create correlations
between two or more variables by connecting
them with a connection arrow.

History and Development of an FBD
• A functional block diagram is also known as a functional flow diagram. As
its name implies, it is a step-by-step representation of a functional flow
that helps to simplify work processes and create a better understanding of
them. The idea was given by Frank Gilbreth in 1921, preceded by other
engineers and scientists who developed a multi-tier process model to
simplify multiple functions and the relationships between them.
• The latest functional block diagram continued to develop in the 1960s
until NASA intervened and leveraged the concept to visualize and
represent the time sequence of units in space systems
• And now, the Functional Block Diagram holds an advantageous position
and is being widely used in various fields of Business Process
Redesign, Business Process Management, Computer System Engineering,
and System Engineering.

Functional Block Diagram Fundamentals
• Though a functional block diagram simplifies the
work processes, breaks down a huge process into
smaller units, and helps us understand the
relationship between two or more variables, it could
still be trickier to understand and interpret the
model. Thus, for your ease and convenience, we
have mentioned the basics of an FBD.
• All the functions are put in a functional block which
is demonstrated by a box. A square box is a symbol
of a function, as illustrated below.

Functional Block Diagram Fundamentals
• A functional block can have two or more two inputs and
outputs. All these inputs and outputs can be connected
with other inputs and outputs of the other functional
block, thus establishing a relationship between the
output of one function and the input of another as
illustrated by the diagram below.
• Functional blocks are standard but can be customized.
Since you will be using the same functional block in your
PLC program, you can use a functional block specific to
one function and use it several times in other instances.

Types of Function Blocks
• Bit Logic Function Blocks
• The basis of a function block is “logic” and is
known to be the simplest form of algorithms.
Within logic, there are two different gateway
mechanisms or logic: AND logic and OR logic.

• Bistable Function Blocks
• Bistable function blocks are known to be the
simplest form of memory. It is up to you if you
want to reset or set an output. The output will
learn and remember the last point of the set
input.

Set/Reset function block (set dominant)

Reset/Set function block (reset dominant)

• Edge Detection
• The next type of function block is Edge
Detection. This type of function block is very
useful and widely being used in PLC
programming and electronics. It got its name
because the input detects a progressive edge
the output will be set. And it gets detected
because the output develops a pulse when a
positive edge is detected.

R_TRIG function block for detecting rising
edge signals

F_TRIG function block for detecting falling
edge signals

• Timer Function Blocks
• They are also being used in PLC engineering
on a wide scale. There are three types of timer
function blocks. These types of blocks include
an on-delay timer, off-delay timer, and pulse
timer. You will need to use only one timer and
derive all the timers out of that timer.

Pulse Timer (TP) function block

On Delay Timer (TON) function block

Off Delay Timer (TOF) function block

• Counter Function Blocks
• The fact about the counter function block is
that it takes inputs and outputs and contains
other types of data. There are three types of
Counter Function Blocks. These types include
Up Counter, Down Counter, and Up-Down
Counter blocks.

Up Counter (CTU) function block

Down Counter (CTD) function block

Up Down Counters (CTUD) function block

Various Communication Protocols in PLC
• Communication is an important role player
in PLC. Apart from traditional hardware IO’s,
communication protocols provide more
diversity and flexibility to exchange data with
various means.

Communication Protocols in PLC
• You must be familiar with OSI layers and
different types of communication hardware
ports used in PLC, to understand the whole
cycle of communication in a better way.

Ethernet/IP
• It is a protocol that works under Common Industrial
Protocol (CIP), which is an open application layer
protocol. It is an advanced version of standard
Ethernet, which is useful only for in-home and
commercial purposes; but not for industrial
applications.
• It defines all the devices on a network as a series of
objects and binds all of them to work on the same
standard. This protocol was developed by Rockwell
Automation.

Ethernet/IP
• Similar protocols with these standards
are Profinet, ControlNet, DeviceNet and
RAPIEnet. They are different in it’s OSI layers
and have their individual functions which
differentiate themselves from each other.
They have been manufactured by different
vendors.

Modbus
• Modbus is a protocol that is used for transmitting
information over a serial lines or Ethernet, based on
master-slave technology.
• Here we have two types of devices in this
communication. The devices which request information
are called Modbus Master and the devices which
provide the information are called Modbus Slaves.
• It is further categorized into Modbus RTU, Modbus
ASCII, and Modbus TCPIP. This protocol was developed
by Modicon (now Schneider Electric).

Profibus
• It is similar to Modbus RTU, which also works
on serial line communication. The only
difference is that it is owned by Siemens
Automation.

DF-1
• It is an asynchronous byte oriented protocol
that is used to communicate with only
Rockwell devices based on RS-232.

Interbus
• It is a serial network protocol which works on
RS-232/RS-485 and works on RTU standard.
• It was developed by Phoenix Contact.

HostLink
• It is a serial network protocol that works on
RS-232/RS-485 and works on RTU standard.
• It works only on Omron PLCs. It was
developed by Omron.

Data Highway (DH+)
• It is a protocol developed by Rockwell Automation and uses
transformer-coupled differential signals; meaning that the transmitter
and receiver stations need not be at the same ground potential.
• It works with the differential signaling concept. It uses two wires for
data transfer and the data is represented with voltage differences in the
two wires. Here the data is carried with differential voltages, the noise
will be easily removed in the two wires.
• It uses half-duplex transmission for communication. It is a very old
protocol and is used only in Rockwell PLCs. Nowadays, it has become
obsolete and rarely used.
• It works on token-passing protocol and uses trunk lines with drops.

Point to Point (PP)
• As the name suggests, it is a communication
protocol which is used to communicate
between only two connected devices.
• It is byte-oriented and is full duplex.

Actual Sensor Interface (ASI)
• It is a protocol that is used to connect all the
sensors and actuators in a field with a single
two-conductor cable.
• It reduces the wiring and manpower done to
connect the field equipment to the PLC. It
works in master-slave technology

Open Smart Grid Protocol
• This protocol has been developed to connect all the
electrical devices in a power grid through
communication and make it a smart grid system.
• The devices are meters, direct load control
modules, solar panels, gateways, and other smart
grid devices.
• Due to use of communication, all the information is
transmitted in a safe and efficient way; to make the
whole power grid system a better way to operate.

CAN (Controller Area Network) Open
• This protocol is an application layer protocol that communicates
with various devices using peer messaging. It is a multi-master
slave communication system and has an object dictionary that
contains all the functions of a device.
• This is an actual communication mean and has standard
communication objects for real-time data (PDO’s), configuration
data (SDO’s), timestamp, sync message, emergency message,
boot-up message, NMT message, and error control message,
and other data.
• In a hardware configuration, one point to remember is that it
requires termination at the end device to establish the whole
link communication.

HART (Highway Addressable Remote
Transducer)
• HART is a protocol in which digital data is
superimposed on the traditional analog signal of
4-20 mA; so that the user gets both the analog
information as well as digital information.
• HART-enabled field transmitters and actuators
are of great use in industrial automation; as the
user gets all the information and calibration
settings sitting at one corner in the office.

PLC Basics and Advanced standard EGY.pptx

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