PLC: Software, Applications
Jason Cardell
November 21, 2014
What is a PLC?
“A Programmable Logic Controller (PLC) is an industrial computer control
system that continuously monitors the state of input devices and makes
decisions based upon a custom program to control the state of output
Overview: History and Current State
 Prior to PLCs, industrial control systems were
designed using relays, cam timers, drum
sequencers, and dedicated closed loop
controllers. Such control systems required a
significant amount of space and were
expensive to design, maintain, and update.
The first PLC was designed and sold By
Modicon (MOdular DIgital CONtroller) in
1977. (Wikipedia)
 The first PLC, model 084, was invented by
Dick Morley (AMCI) in response to a need
for more flexible control systems in the
automotive industry. As car designs evolved,
manufacturing control needed to evolve.
Overview: Current State - Software
 PLCs are generally programmed using PC based software packages.
 Once a control system has been designed in the software application and compiled,
the program can then be transferred to the PLC’s onboard non-volatile memory.
 PLC manufactures typically have a proprietary software package that communicates
with their designed hardware. For example, if you are using an Allen-Bradely PLC
then you will be required to use a compatible version of RSLOGIX to
communicate with it.
 Although each PLC manufacturer designs their own software as a programming
GUI, PLC language standards have been established (IEC 61131) and are strictly
adhered to in industry.
 All PLC software packages can be used to program in the 5 basic PLC languages
that will be covered later in this power point.
Overview: Current State – Software Packages
Overview: Current State - Languages
 Ladder Logic (LD)-Traditional ladder logic is a graphical programming
language. Initially programmed with simple contacts that simulated the opening
and closing of relays, Ladder Logic programming has been expanded to include
such functions as counters, timers, shift registers, and math operations (AMCI).
 Ladder Logic is the most
commonly used PLC
programming language in
 Ladder Logic uses a digital
version of relay logic and
thus relay circuits could
easily be converted to this
digital form.
Overview: Current State - Languages
 If (SW1 OR SW2) and OVR1 =1, THEN START =1
Switch 1 (NOC)
Override 1 (NOC)
Start Bit(Coil)
Subtract (FB)
SPEED = X65 – 1
X54 = 1
Pump1 (NOC)
Enable (NOC)
Output 100 (output coil)
%Q designates that %Q100
Is a physical output from PLC
Overview: Current State - Languages
 Ladder Logic (LD)- Click (PLC Software for our LAB)
Overview: Current State - Languages
 Ladder Logic (LD) – Every programs Ladder diagramming looks a little
different. Tri-PLC has combined LD with higher Languages such as C++.
Overview: Current State - Languages
 Function Block Diagram (FBD) - A graphical language for depicting signal and
data flows through re-usable function blocks. FBDs are very useful for
expressing the interconnection of control system algorithms and logic.(AMCI).
 Function Block Diagrams are often coupled with Ladder Logic to increase
readability and improve diagnostic efficiency.
Overview: Current State - Languages
 Structured Text (ST) – A high level text language that encourages structured
programming. It has a language structure (syntax) that strongly resembles
PASCAL and supports a wide range of standard functions and operators
(AMCI). Some software packages allow programming in higher languages such
as C++.
 Structured Text offers easy access to and manipulation of Ints, Dints, Reals,
and Strings. Although this same code could be programmed in LD or one of
the other languages, implementation is simpler and more compact in ST.
Overview: Current State - Languages
 Instruction List (IL): A low level “assembler like” language that is based on
similar instructions list languages found in a wide range of today’s PLCs (AMCI).
Overview: Current State - Languages
 Sequential Function Chart (SFC) A
method of programming complex
control systems at a more highly
structured level. A SFC program is an
overview of the control system, in
which the basic building blocks are
entire program files. Each program
file is created using one of the other
types of programming languages. The
SFC approach coordinates large,
complicated programming tasks into
smaller, more manageable tasks.
Overview: Where Used?
 Modern theme parks are riddled
with PLCs that constantly monitor
rides, such as roller coasters, to
ensure the safety of passengers
and warn of possible failures
before catastrophe strikes. .
Overview: Where Used?
 Automated manufacturing lines
predominately use PLCs to manage their
machine controls. PLCs offer a quick flexible
environment for Automation Engineers to
design in.
 AS/RS control systems are often
implemented using PLC based controls.
 Asynchronous control systems in industry
typically use PLCs to interpret input devices
and control output devices based on the given
inputs. In many PLC applications, a
combination of asynchronous and
synchronous controls are used.
Overview: Where Used?
 PLCs offer power plants the ability to
constantly monitor dangerous systems
remotely and provide maintenance workers
with advanced diagnostic capabilities.
 Water treatment facilities rely on the
dependability and repeatability offered by
PLC based control systems. PLCs monitor
and control pneumatic valves, pipe
pressures, and flow rates via field bus
communication networks.
Overview: When Used?
 PLCs are used in virtually every industry and are the
major means of controlling any industrial grade
automated system.
 PLCs are used when a flexible control system is
needed that can be easily updated to incorporate future
technologies and accessed by maintenance workers for
ease of troubleshooting.
Overview: Cost per license
Rslogix 500
GX Developer
Simatic Step 7
Rslogix 5000
Cost (Low) Cost (High)
*The five basic PLC programming languages outlined in Standard IEC 61131 are available
in any of the above mention software packages. Namely: IL, ST, FBD, LD, SFC.
Overview: Supporting Technologies
Sensors, Switches
PLC Software
Light Curtains / Safety Devices
Digital / Analog I-O Blocks
PLC Hardware
Encoders, Actuators, Contacts
Application Rules & Limitations (IEC 61131)
 IEC (International Electrotechnical Commission) 61131-1: General
 IEC 61131-2: Equipment test requirements
 IEC 61131-3 is the international standard for programmable controller
programming languages. As such, it specifies the syntax, semantics and display
for the following suite of PLC programming languages: (Rockwell Automation)
 Ladder diagram (LD)
 Sequential Function Charts (SFC)
 Function Block Diagram (FBD)
 Structured Text (ST)
 Instruction List (IL)
 IEC 61131-4: User guidelines
 IEC 61131-5: Communications
Application Limitations
 Cost is one of the largest limitations in PLC use. In order to implement a
robust control system using a PLC you need PLC hardware, PLC software, an
array of sensors, switches, contacts, safety devices, etc. and a great deal of
programming design work.
 Time limitations often play an important role in control system design.
Building a robust PLC based control system requires a great deal of upfront
programming time. However, when a flexible robust control system is needed,
PLCs are worth the investment of time.
 Hardware Compatibility: Some control systems require hardware that
communicates best with a PC. PLCs can be inefficient at communicating with
and controlling hardware meant for a pc interface. Specific subsystem hardware
may limit the control you have from a PLC and may require a PC type control
or a hybrid control system that uses a PLC and PC.
Primary Vendors of Technology
How Automation Integrates Process
 Wherever you mix automation devices, you often use PLC to manage
the asynchronous (event) state between the devices.
Application Article Summary
 Advantages to PLC use over other industrial control systems
 Flexible and can be redesigned to control other systems quickly and easily.
 Computational abilities allow more sophisticated control.
 Trouble shooting aids make programming easier and reduce downtime.
 Early PLCs
 Early PLCs were programmed using proprietary programming panels and offered only Ladder
Logic as the programming language with simple coils and contacts.
 PLCs vs. PCs
 Modern PLCs are approximately equivalent to desktop computers but run on more stable
operating systems with control hardware that is more robust.
 PCs are generally not accepted in heavy industry due to hardware that has lacking levels of
tolerance to temperature, humidity, vibration, and longevity in comparison to PLC hardware.
 PLRs (Programmable Logic Relays) are cheap versions of PLCs with reduced reliability and
Application Article Summary
 The following topics are covered in more detail in the article
 A history of PLCs.
 PLCs where created in response to a call for more flexibility from the automotive industry.
 Scan time estimations of PLCs
 A comparison between PLCs and other control systems
 Digital/analog signal handling
 Overview of Ladder Logic and reasons for its use
Application Example
 Design a PLC based control system, using a combination of two or more
languages, outlined in IEC 61131-3, that will flash at least one LED light at two
different rates based on no more than two inputs to the PLC.
 You are limited to two PLC inputs and two PLC outputs.
Solution – Ladder Logic (LD)
Solution – Structured Text (ST)
Solution – Structured Text (ST)
PLCs continuously monitor inputs and control outputs based on program logic and input values.
PLCs were designed in response to a need for more flexible control systems in the automotive
industry. As cars evolved, there was a need for the manufacturing control system to evolve.
The five standard PLC languages are: Ladder Logic(LD), Function Block Diagram (FBD), Structured
Text(ST), Instruction List(IL), and Sequential Function Chart (SFC).
PLCs are used in every industry and are the primary means of control for asynchronous systems.
The cost of PLCs and PLC software packages varies greatly based on vendor and licenses needed.
Although all software packages offer the five basic languages, some vendors offer built in function
blocks, higher level programing (C++, Java, object oriented, etc.), and other features.
PLC hardware requires specific proprietary PLC software in order to program.
IEC 61131 defines PLC hardware and software standards.
Major software / application limitations include: Cost, Time and Hardware compatibility
Automation Examples
Additional References

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