In this article we’ll look at basic PLC architecture, the different types of PLCs used in industrial automation, advantages and disadvantages of PLC types and compare them in a handy reference table.
What Is PLC Architecture?
The term PLC architecture refers to the design specification of the various PLC hardware and software components and the how they interact with one another to form the overall PLC system. The architecture of a PLC is based on the same principles of that used in standard computer architecture. However, PLC architecture does differ because the design is based around providing high reliability, immunity to harsh industrial environment, ease of maintenance and access to large amounts of peripheral inputs and outputs.
If a PLC system is described as having closed architecture it refers to a proprietary system of hardware and software components that cannot (or is difficult to) connect to other manufacturers components and software. However if a PLC system is described as having open architecture it refers to the PLC system as having off the shelf components that adhere to a common standard and are easily connected to other manufacturer’s hardware and software components.
The Basics of How PLC Architecture Works
The heart of the PLC system is the CPU (Central Processing Unit). It is made up of a control unit and processor. The CPU control unit manages the interaction between the various PLC hardware components while the CPU processor handles all the number crunching and program (eg ladder logic) execution.
The block diagram below explains the basic architecture of a PLC….

Data flow is from the input devices, through the CPU processor and then to the output devices. The CPU processor also exchanges data with the program and data memory. Once all the data is gathered the program (eg ladder logic) is processed in a cyclic fashion. The resulting data flows to the output interface for conditioning and execution of the output devices.
The CPU also controls and exchanges data with the communication interface and devices.
An addressing system is used for data organization that is shared between the various hardware components.
A programming terminal is used to formulate the PLC program (eg ladder logic), load the program into the controller and monitor/control the PLC and its program.
The power supply is responsible for supplying and managing the power requirements of the various PLC hardware components.
Types of PLC Architecture
PLC architecture has advanced tremendously since its inception of the PLC back in the 1960s. Today the options and variations available to the traditional PLC architecture types is almost endless between different PLC manufacturers.
The 3 distinct types of PLC architecture available for use in industrial automation are known as fixed, modular and distributed. The terminology surrounding PLC types can vary between PLC manufacturers, especially when talking about fixed PLCs. There is also crossover between PLC types with some fixed type PLCs having modular type features and some modular type PLCs having distributed type features.
When considering PLC architecture types we can generally say that fixed PLCs are used for smaller sized, less complex application. Modular PLCs are generally used for medium sized, more complex applications. Whereas distributed PLCs are used for large sized, extensive application spread across multiple location.

Let’s take a look at the 3 types of PLCs in more detail….
Fixed PLC Type Architecture
A fixed PLC employs single unit architecture where all the hardware components are embedded into a single unit. Hardware components such as the power supply, CPU, memory, input, output and communication interfaces are all are built into a fixed PLC. The most common names given to fixed PLCs by different manufacturers are fixed, integrated, nano, micro, compact, small, mini, basic, unitary, standard and brick.

Some examples of fixed PLC types by different manufacturers and the terminology they use is shown below:
- Allen Bradley PLC – Micro
- Omron PLC – Compact
- Siemens PLC – Basic
- Delta PLC – Standard
- Koyo PLC – Brick

Regardless of the different terminology that PLC manufacturers use to identify the type, size and performance level of their PLCs, a fixed PLC is universally characterized by:
- PLC naming convention – Fixed, integrated, nano, micro, compact, small, mini, basic, unitary, standard and brick.
- CPU processor – Low powered.
- Program and data memory size – Small.
- Power supply – Embedded.
- Input Interface – Embedded.
- Output Interface – Embedded.
- Communication interface – Embedded.
- Mounting system – Single unit.
- Physical size – Small.
- Flexibility – Input, output and communication interfaces are fixed.
- Expandable – No
- Customizable – No
- Applications – Basic applications with small number of inputs and outputs.
- Cost – Low $$$$$.
Advantages of Fixed PLCs
Fixed PLCs have some great advantages over other types of PLCs. They have been specifically design to cater for smaller, low end automation projects. The advantages of a fixed PLC are:
- Small in size so they do not take up very much space in an enclosure.
- Quick and easy to mount.
- Low in cost so they are an economical solution for basic applications.
Disadvantages of Fixed PLCs
If you’re considering selecting a fixed PLC to automate your next application its prudent to also consider some of disadvantages to make sure you are selecting the type of PLC that is appropriate for your application. The disadvantages of a fixed PLC are:
- CPU processing power is low and memory is small so complex tasks can be difficult to realize.
- Inflexible because the number of input, output and communication interfaces are fixed.
- Only suitable for basic applications with small number of inputs and outputs.
The Modern Day Fixed PLCs
The majority of modern day fixed PLCs are far from what the traditional definition of a fixed PLC is. Technology advancements and demand from industry has driven them away from being a simple, low powered unit with fixed amount of input, output and communication interfaces.
The modern day fixed PLC is feature packed with a large range of options such as powerful CPU’s, expandable modules and network capabilities that are bordering and even overlapping into modular and distributed PLC architecture. This has resulted in the fixed PLC expanding its reach into a larger range of industrial automation applications.
For example the Omron Sysmac CP1L and Koyo DL06 PLCs have traditional fixed PLC architecture but at the same time are expandable with built in slots to allow for extra plugin modules. Another example of a modern day fixed PLC that incorporates modular PLC design is the Delta DVP ES2 with a rack-less design and clip-in expansion I/O modules. The Allen Bradley MicroLogix 1100 PLC range also has the ability to further extend using I/O expansion modules to the side of the PLC using ribbon cable…..

Modular PLC Type Architecture
A modular PLC has a separate module for each of its hardware components. Each PLC module is interconnected using a common mounting system. The mounting system has a certain amount of modules that it can accommodate. This means a modular PLC can be configured to be application specific.

A PLC module is a hardware component that carries out a specific function in accordance to the architecture of the PLC system. The main modules used in a modular type PLC are the processor module, power supply module, input modules, output modules and communication modules. The design of these modules will vary for different manufacturers and usually cannot be interchanged between PLC manufacturers.
Modular PLCs are used for automating industrial applications where a higher powered processor and large numbers of input and output devices are required. These types of applications that use modular PLCs are usually associated with a higher level of complexity with regards to operation, process control and monitoring. Some examples of industries that readily use modular PLCs are manufacturing, food and beverage, mining and logistics.
Some examples of modular PLC types by different manufacturers are shown below:

A modular plc is characterized by:
- PLC naming convention – Modular.
- CPU processor – Medium to High powered.
- Program and data memory size – Medium to Large.
- Power supply – Module.
- Input Interface – Input Module.
- Output Interface – Output Modules.
- Communication interfaces – Communication Modules.
- Mounting system – Rack, backplane, rail or chassis.
- Physical size – Medium to Large.
- Flexibility – High
- Expandable – Yes
- Customizable – Yes
- Applications – Medium to High end applications with large number of inputs and outputs.
- Cost – Medium to high $$$$$.
What are the Advantages of Using a Modular PLC Instead of a Fixed PLC?
Modular PLCs have some great advantages over other fixed PLCs. They have been specifically design to cater for medium to high end automation projects.
The advantages of using a modular PLC instead of a fixed PLC are that modular PLCs have lager memory, higher performance processors, larger number of input and outputs, increased communication options, are fully customizable and are easily expanded. This enables the modular PLC to handle larger scale applications and of higher complexity compared to a fixed PLC.
Modular PLCs also have the ability to have remotely mounted input and output modules (distributed I/O) that are interconnected using a communication link. This allows for increase number of inputs and outputs, reduced cable requirements and installation flexibility.
The modular PLC also has maintenance advantages over fixed PLCs. Each hardware component is separate housed in a module which can be replaced if it is faulty. Whereas a fixed PLC has all its component embedded into a single unit. So when there is a fault the whole unit must be replaced in its entirety and the faulty unit ends up in the trash.
Disadvantages of Modular PLCs
If you’re considering selecting a modular PLC to automate your next application it’s wise to also consider some of disadvantages to make sure you are selecting the most suitable PLC for your application. The disadvantages of modular PLC are:
- Large in size so they take up more space in an enclosure than a fixed PLC.
- The mounting system is more complex than a fixed PLC.
- Higher in cost than a fixed PLC so may not be cost effective for smaller applications.
Distributed PLC Type Architecture
A distributed PLC is a high end PLC system with modular architecture and the capability to interconnect hardware components across different locations via high speed communication links. Each location in the distributed PLC system contains multiple hardware modules that are housed in a mounting system and are usually called a node, rack or drop.
Each drop, node or rack in the Distributed PLC system must have a communication module and can either contain a PLC processor module with input and output (I/O) modules or just I/O modules. When there is a communication module with no PLC processor module and just I/O modules then the node is called distributed I/O or remote I/O.

Distributed PLCs are used for large factories and large processing facilities because they are not limited to physical location. They allow hardware components to be located in different locations by utilizing high speed communication links to interconnect processors and distributed I/O. The distributed PLC type is considered to be a site wide process control solution.
The biggest differences between distributed PLCs and other types of PLCs is that they contain high performance processors, large memories and are able to handle large volumes of I/O, use higher level programming languages and can handle large amounts of complex process control tasks.
In the past a Distributed Control Systems (DCS) was used for large process plants. But the technology driven PLCs of today are performance packed and can handle the heavy demands of a distributed control system.

In today’s world of advanced PLCs most modular PLCs have the features of a distributed PLC. Regardless of the terminology used a distributed PLC is universally characterized by:
- PLC naming convention – Distributed or Modular with Remote I/O.
- CPU processor – High powered.
- Program and data memory size – Large.
- Power supply – Module.
- Input Interface – Input Modules.
- Output Interface – Output Modules.
- Communication interfaces – Communication Modules.
- Mounting system – Rack, backplane, rail or chassis.
- Physical size – Medium to Large.
- Flexibility – High
- Expandable – Yes
- Customizable – Yes
- Applications –High end applications with large number of inputs and outputs.
- Cost –High $$$$$.
What are the Advantages of Using a Distributed PLC?
Distributed PLCs have been designed to process and handle large amounts of data and complex process control. As such they have the following advantages over other PLC types….
- Plant wide control network with multiple processors and remote I/O drops.
- High performance processor.
- Large program and data memory.
- Able to handle large volumes of I/O.
- Can handle large amounts of complex process control tasks.
- Ease of maintenance.
- Save time and money on installation costs.
Disadvantages of Distributed PLCs
Because of the high end features that a distributed PLC has they come with some disadvantages that must be considered when selecting the type of PLC that is required for your automation system….
- Large in size with bigger installation footprint.
- The mounting system is more complex than a fixed PLC.
- Higher in cost than other types of PLC so they may not be cost effective for smaller less complex applications.
- Higher level programming skills may be required.
PLC Types Comparison Table
PLC TYPES | FIXED PLC![]() | MODULAR PLC![]() | DISTRIBUTED PLC![]() |
CPU Performance | Low | Medium to High | High |
Program & Data Memory Size | Small | Medium to Large | Large |
Power Supply | Embedded | Module | Module |
Input Interface | Embedded | Modules | Modules |
Output Interface | Embedded | Modules | Modules |
Communication Interface | Embedded | Modules | Modules |
Mounting System | Single Unit | Rack, Back plane, Rail or Chassis. | Rack, Back plane, Rail or Chassis. |
Physical Size | Small | Medium to Large | Medium to Large |
Flexibility | No | Yes | Yes |
Customizable | No | Yes | Yes |
Applications | Basic applications with small number of inputs and outputs. | Medium to high end applications with large number of inputs and outputs. | High end applications and plant wide control with a very large number of inputs and outputs. |
Cost | Low $$$$$. | Medium $$$$$. | High $$$$$. |
In the next section we’ll learn about PLC hardware and the essential components that make up a PLC system.
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FAQs
What are the types of PLC architectures? ›
The 3 distinct types of PLC architecture available for use in industrial automation are known as fixed, modular and distributed. The terminology surrounding PLC types can vary between PLC manufacturers, especially when talking about fixed PLCs.
What is PLC and its architecture? ›PLC architecture
It consists of a central processing unit (CPU) containing the system microprocessor, memory, and input/output circuitry. The CPU controls and processes all the operations within the PLC.It is supplied with a clock that has a frequency of typically between 1 and 8 MHz.
There are two types of PLCs- Compact PLC and Modular PLC. • In Compact PLC, the capability of the I/O module is fixed. • In Modular PLC, the capability of the I/O module is not fixed.
What is PLC ladder logic? ›Ladder logic is a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logic hardware. It is primarily used to develop software for PLCs used in industrial control applications.
How many types of PLC logic are there? ›PLCs can be divided into two main types depending on their power supply: those with a built-in power supply and those that require an external power supply.
What are the 3 types of PLC? ›PLC are divided into three types based on output namely Relay output, Transistor output, and Triac Output PLC. The relay output type is best suited for both AC and DC output devices.
What are the 5 most popular type of PLC? ›- Ladder Diagram (LD)
- Sequential Function Charts (SFC)
- Function Block Diagram (FBD)
- Structured Text (ST)
- Instruction List (IL)
PLC components
A typical PLC is connected to a power supply and consists of a central processing unit (CPU), a mounting rack, read-only memory (ROM), random access memory (RAM), input/output (I/O) modules, a power supply, and a programming device.
PLC stands for Programmable Logic Controller. They are industrial computers used to control different electro-mechanical processes for use in manufacturing, plants, or other automation environments. PLCs vary in size and form factors.
What type of device is PLC? ›A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis.
What is PLC in PDF? ›
Page 1. Programming Logic controllers. Programmable Logic Controller (PLC) is a microprocessor based system that uses programmable memory to store instructions and implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes. Page 2.
What is PLC and its application? ›Programmable logic controllers (PLCs) are integral to industrial automation and industrial process control. PLCs manage various actuators, analog, and digital sensors, and communicate complex interfaces in changing protocols. Other than control functions, PLCs execute data conversion and signal processing.
What are the 4 main components of a PLC? ›The main components of a PLC consist of a central processing unit (CPU), power supply, programming device, and input and output (I/O) modules. The CPU is the brain of the PLC and carries out programmed operations. These operations or outputs are executed based on signals and data provided from connected inputs.
How does PLC logic work? ›The programmable logic controller will process the given information (inputs), execute the instruction from the program, and deliver the results (outputs) based on the provided information and written logic. PLCs operate in a repeating loop once the user determines the inputs and outputs.
Which logic is used in PLC? ›Most PLC programming software offers programming in Ladder Logic, or “C”. Ladder Logic is the traditional programming language. It mimics circuit diagrams with “rungs” of logic read left to right.
Which language is used in PLC? ›Ladder logic is the most common programming language used for programmable logic controllers (PLCs) in the U.S. Instruction list, function block diagram, structured text, and sequential function charts are all useful programming languages and may be more appropriate than ladder, depending on the application.
What is PLC block diagram? ›PLC takes input instructions in the form of ladder diagram or computer software instructions. These instructions are decoded in CPU and CPU provides differed signals to control or to operate many devices of system. When these devices change their position or cause to change controlled variable.
What are the 6 main components of a PLC? ›- Central processing unit.
- Rack or mounting.
- Input assembly.
- Output assembly.
- Power supply.
- Programming unit.
A product life cycle consists of four stages: introduction, growth, maturity, and decline.
What are 4 PLC languages? ›- Ladder Logic.
- Function Block Diagram.
- Sequential Function Charts.
- Structured Text.
- Instruction List.
What are the five applications of PLC? ›
- 1) Road Traffic Signals. It's difficult to picture our modern world without traffic signals. ...
- 2) The Automatic Car Wash. ...
- 3) The Elevator. ...
- 4) Automatic Doors. ...
- 5) Conveyer Belts. ...
- 6) Roller Coasters.
- 6 Essential Characteristics of a PLC.
- Shared mission, vision, values, goals. ...
- Collaborative teams focused on learning. ...
- Collective inquiry. ...
- Action orientation and experimentation. ...
- Commitment to Continuous improvement. ...
- Results orientation.
- What do we want all students to know and be able to do?
- How will we know if they learn it?
- How will we respond when some students do not learn?
- How will we extend the learning for students who are already proficient?
PLC programmers work in the manufacturing industry where they program automated machinery using programmable logic controllers (PLC). They create design schematics, write and install computer programs, and test the efficacy of the installed program. Completely free trial, no card required.
Who controls a PLC? ›In a PLC, shares are sold to the public on the stock market . People who own shares are called 'shareholders'. They become part owners of the business and have a voice in how it operates. A chief executive officer (CEO) and board of directors manage and oversee the business' activities.
Which is the first PLC model? ›Modicon PLC. Following this success, Bedford Associates changed its name to Modicon PLC. They began producing the Modicon 084, the first PLC.
How many architectures are there? ›6 Different Types of Architecture: Quality, Characteristics, and Uses.
What are those five 5 types of PLC programming languages? ›- Ladder Logic.
- Function Block Diagram.
- Sequential Function Charts.
- Structured Text.
- Instruction List.
PLC components
A typical PLC is connected to a power supply and consists of a central processing unit (CPU), a mounting rack, read-only memory (ROM), random access memory (RAM), input/output (I/O) modules, a power supply, and a programming device.
Typically a PLC system has five basic components. These are the processor unit, memory, the power supply unit, input/output interface section and the programming device.
What are the 4 layers of architecture? ›
Although the layered architecture pattern does not specify the number and types of layers that must exist in the pattern, most layered architectures consist of four standard layers: presentation, business, persistence, and database (Figure 1-1).
What are the four types of system architecture? ›- Hardware architecture.
- Software architecture.
- Enterprise architecture.
- Collaborative systems architectures(such as the Internet, intelligent transportation systems, and joint air defense systems)
- domestic architecture.
- religious architecture.
- governmental architecture.
- recreational architecture.
- commercial architecture.
- 1 1. Residential architecture.
- 2 2. Commercial architecture.
- 3 3. Landscape architecture.
- 4 4. Interior design architecture.
- 5 5. Urban design architecture.
- 6 6. Green design architecture.
- 7 7. Industrial architecture.
The American Institute of Architects (AIA) defines Five Phases of Architecture that are commonly referred to throughout the industry: Schematic Design, Design Development, Contract Documents, Bidding, Contract Administration.
What are the 3 rules of architecture? ›Firmitas (Firmness, Durability) – It should stand up robustly and remain in good condition. Utilitas (Commodity, Utility) – It should be useful and function well for the people using it. Venustas (Delight, Beauty) – It should delight people and raise their spirits.
What are the 6 types of programming language? ›- C.
- Java.
- Python.
- C++
- C#
- Visual Basic.
- JavaScript.
- PHP.
There are three types of programming languages: machine language, assembly language, and high-level language.