+91 981-125-3572 info@innovicindia.com

Discussion Topics

  • Introduction to PLC
  • Definition of PLC
  • PLC Structure
  • Soft Wiring
  • Basic Components Of PLC
  • Communication Protocol
  • Scan Cycle & Scan Time
  • PLC Advantages
  • PLC Programming Languages
  • Representation Of
  • Field/Electrical & PLC Contacts
  • Types of PLC
  • Behavior of Contacts
  • PLC Sourcing & Sinking
  • Redundancy


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

Other engineering firms developed their own versions of this device, and it eventually came to be known in non-proprietary terms as a PLC, or Programmable Logic Controller.

The purpose of a PLC was to directly replace electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks.

A PLC has many “input” terminals, through which it interprets “high” and “low” logical states & Analog values from switches and sensors.

It also has many output terminals, through which it outputs “high” and “low” signals to power lights, solenoids, contactors, small motors, and other devices lending themselves to on/off control and also analog output for controlling control valves, motor speed control etc.

In an effort to make PLCs easy to program, their programming language was designed to resemble ladder logic diagrams. Thus, an engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions.


A Programmable Logic Controller also called a PLC or programmable controller is a computer-type device used to
control equipment in an industrial facility.

PLC is a Userfriendly, Microprocessor Based Industrial Computer.

It behaves like an Interface Between Field I/P devices and Field O/P devices.

A PLC is an industrial computer that reads the inputs of a process and makes decisions based on the programmed logic to control the outputs of a process.

The kinds of equipment that PLCs can control are as varied as industrial facilities themselves. Utility Plants, Batch Control Applications, Chemical Processing, Conveyor systems, food processing machinery, auto assembly lines, etc…you name it and there’s probably a PLC out there controlling it.


In a traditional industrial control system, all control devices are wired directly to each other according to how the system is supposed to operate. In a PLC system, however, the PLC replaces the wiring between the devices.

Thus, instead of being wired directly to each other, all equipment is wired to the PLC. Then, the control program inside the PLC provides the “wiring” connection between the devices.

The control program is the computer program stored in the PLC’s memory that tells the PLC what’s supposed to be going on in the system. The use of a PLC to provide the wiring connections between system devices is called soft wiring.

PLC Advantages:

In addition to the programming flexibility we just mentioned, PLCs offer other advantages over traditional control systems.

These advantages include:

  • high reliability
  • small space requirements
  • computing capabilities
  • reduced costs
  • ability to withstand harsh environments
  • expandability

Basic Components Of PLC:

A PLC consists of the required quantities of the following types of modules or cards, mounted on a common physical support and electrical interconnection structure known as a rack.

A typical PLC rack configuration is shown in below figure.

  1. Power supply
  2. Processor
  3. Input/Output (I/O)
  4. Communication Module
  5. Communication Media and Protocols
  6. Programming Device & Software

System Architecture/ Block Diagram Of PLC:

1. Power Supply

The power supply converts facility electrical distribution voltage, such as 230 VAC, 120 VAC or 125 VDC to signal-level voltage used by the plc processor and other modules.

The Power Supply is connected to AC mains for the supply voltage.

The output of the Power supply is a DC voltage used to power all of the other modules associated with the PLC.

The Power supply DOES NOT provide power for field devices

2. Processor:

The Processor consists of the CPU (central processing unit) and memory.

The processor of a Programmable Logic Controller is the section that organises all control activity by receiving inputs, making logical decisions based on the program, and controlling the outputs.

The processor section makes decisions needed to observe and operate the field devices connected to the Input/Output modules.

The decisions are based upon a user-created program saved in the memory. The memory also stores data representing the condition of all input field devices and contains the data telling the output field devices what to do.

CPU(The Central Processing Unit):

The central processing unit (CPU) is the part of a programmable controller that retrieves, decodes, and processes information.

It also executes the control program stored in the PLC’s memory. In essence, the CPU is the “brains” of a programmable controller.

It functions much the same way the CPU of a regular computer does, except that it uses special instructions and coding to perform its functions.


Memory system is the section of the CPU that stores both the control program and data from the equipment connected to the PLC.

3. Input/Output Module:

The input/output (I/O) system is the section of a PLC to which all of the field devices (like flow transmitters, pressure transmitters, control valves, analyzers, substation feeders for motor control etc.) are connected.

If the CPU can be thought of as the brains of a PLC, then the I/O system can be thought of as the arms and legs.

The I/O system is what actually physically carries out the control commands from the program stored in the PLC’s memory.

The I/O system consists of two main parts:

  • The Rack
  • I/O modules

The Rack: The rack is an enclosure with slots in it that is connected to the CPU.
I/O modules: I/O modules are devices with connection terminals to which the field devices are wired.

Together, the rack and the I/O modules form the interface between the field devices and the PLC.

Field Input & Field Output Devices:

The Field Input devices in PLC (Programmable Logic Controller) refer to the required components to take the data or pass the data from the physical world (i.e. field data) to the PLC. Input devices can be Transducer, Sensors, Switches, and Push Button which is connected with PLC.

Digital Inputs include Push-buttons, Limit switches, Relay Contacts, Proximity Switches, Photo sensors (On/Off), Pressure switches, etc…

Similarly Analog inputs includes Transducer, Power monitoring devices, RTD, Temperature sensor, Pressure sensors etc.

On the other hand, Field Output devices are which will be controlled or start/stop through PLC output like AC Motors, Conveyors, Lamps, AC drives.

Digital Output includes Indicator Lights, Alarms, Actuators, Solenoid Valves, etc…

An analog PLC output is a time-varying or continuous signal from the PLC CPU to the field output devices. Basically, PLCs provide two types of analog output signals-current and voltage. PLC-system analog outputs—commonly used to control actuators, valves, and motors in industrial environments— employ standard analog output ranges such as 0-10 V, or 4-20 mA.

Types of Digital PLC Outputs:

Digital PLC outputs are classified into three categories namely:

  • Relay PLC Outputs: The relay outputs from PLCs are used to operate both DC (Direct Current) and AC (Alternating Current) field output devices. They provide low resistance control of up to 2A (Amperes).
  • Transistor PLC Outputs: Transistor outputs are voltage-dependent and they can only operate DC loads. PLC control systems providing transistor outputs are mainly used in low-power DC circuitry such as within microprocessors and in applications that require faster-switching operations like controlling lights.
  • TRIAC PLC Outputs: TRIAC is a term that means Triode for Alternating Current (AC). TRIACs are silicon-based, solid-state electronic switches that are activated by a small amount of control voltage from a PLC for example. Their working principle is similar to that of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistors. TRIAC PLC outputs are ideal for controlling low-power AC loads such as motor starters, lighting systems, and contractors

4. Communication Module:

Communications modules are available for a wide range of industry-standard communication network connections. These allow digital data transfer between PLCs and to other systems within the facility.

Most commonly used modules are Modbus communication cards or Serial communication.

Some PLCs have communications capability built-in to the processor, rather than using separate modules.

5. Communication Media and Protocols:

The most common communication media used are copper-wire, coaxial, fiber optics, and wireless. The most common “open” communication protocols are Ethernet, Ethernet/IP, and DeviceNet.

“Open” systems generally provide “plug and play” features in which the system software automatically recognizes and communicates to any compatible device that is connected to it.

Other widely accepted open protocols are Modbus, Profibus, and ControlNet.

6. Programming Device & Software:

The Programming Device in today’s industrial applications is usually a laptop or a desktop computer that facilitates the creation of decision-making programs destined for the PLC by using PLC Programming Software.


The most important working principle is- the PLC is operated by continuously scanning programs. Scanning happens every time per millisecond. So, it is called as the Scan Cycle.

For this scan cycle, PLC required a little amount of time in the range of milliseconds or microseconds.

The scan cycle consists of the following three basic main steps.

  1. Read the inputs
  2. Execute the program by the CPU
  3. Update the output


The amount of time is taken by the processor to read/sense the first input and execute the last output called the Scan time.
PLC is so fast as it can easily scan and execute the program in few milliseconds.

Communication Protocol:

The communication protocol is a set of rules that govern data communication. In other words, the communication protocol acts as an agreement between two or more communicating devices.

By using the communication protocols, two devices connect and communicate with each other. Without communication protocol, devices can only be connected but not communicated.

List of PLC Communication Protocols:

  • EtherNet/IP
  • Profibus
  • Modbus
  • Interbus
  • ProfiNet
  • ControlNet
  • DeviceNet
  • DirectNet
  • CompoNet
  • RAPIENet
  • EtherCAT
  • MelsecNet
  • Optomux
  • DF-1 Protocol
  • HostLink Protocol
  • DH- Data Highway
  • PPI- Point to Point
  • EtherNet Powerlink
  • MPI- Multi-Point Interface
  • EGD- Ethernet Global Data
  • AS-i- Actuator Sensor Interface
  • OSGP- Open Smart Grid Protocol
  • DNP3- Distributed Network Protocol
  • SDS- Smart Distributed System Protocol
  • PieP- Process Image Exchange Protocol
  • SRTP- Service Request Transport Protocol
  • BSAP- Bristol Standard Asynchronous Protocol
  • FINS- Factory Interface Network Service Protocol
  • HART- Highway Addressable Remote Transducer Protocol
  • Recommended Standard (RS-232, RS422, and RS-485) Protocol

These are the foremost communication protocols used for the PLC and other network connections. These protocols are supported by different PLC software brands. The communication protocols are dependent upon three fundamental parts such as baud rate, network length, and the number of nodes. More communication protocols are available in the market. You can easily buy PLC communication protocols.

Baud Rate:

Baud rate is also known as Communication speed.

The rate of data transmission on the network is called Baud Rate. The unit of Baud Rate is bits/second (b/s).
In simple terms, it is calculated as the “number of data/Bits transferred per second”.

Common Values(Speeds) are 1200, 2400, 4800, 9600, 19200, and 38400.

Binary Data Type Conversion for Baud Rate:

Binary system based on the bit. Bit is either 0 or 1.

1 Nibble= 4 Bits
1 Byte= 8 Bits
1 Word= 2 Bytes = 16 Bits
1 Double Word= 4 Bytes= 32 Bits

Let’s see, binary data representation diagram

Resolution in I/O cards in PLCs:

It is the minimum change in i/p parameter which can sensed by the i/p card. As far as Digital I/O is concerned it takes only one bit for operation. In case of analog input, the resolution determines how many bits are used for input or output. For example a 12 bit resolution card means the input will come as 0 to 4096 count (2^12). For 16 bit data, the counts will be from 0 – 65536(2^16). The more the resolution the data will be more accurate.

Characteristics of PLC Communication Protocols:

When PLC modules are connected over the network, standard communication protocols are used.
The different types of standard communication protocols support different speeds (baud rate), distances (network length), and the number of connecting devices (nodes).

Which PLC communication protocol we should use?

  • If we want to communicate with the more number of nodes with more speed, Ethernet is the best choice. You can read detail about ethernet here.
  • If our network has a length of more than 25 Kilo-meter, you should use the Control Net protocol.

Types of PLC

Two types of PLCs are used for commercial or industrial purposes.

  1. Compact PLC
  2. Modular PLC

1. Compact PLC

It is also called as Integrated PLC or Fixed PLC. The compact PLC has a fixed number of input/output modules along with power supply and CPU.

2. Modular PLC

It consists of a variable number of inputs and outputs. Inputs and outputs can be added to the modular PLC systems by the user. If you look at the below PLC designing structure, it looks more like a rack. So, it is also called as Rack-Mounted PLC.

PLC Programming Languages:

Based on the International Electrotechnical Commission(IEC) standard, PLC programming languages are classified into five main standards.

  1. Ladder diagram (LD)
  2. Instruction List (IL)
  3. Structured Text (ST)
  4. Function Block Diagram (FBD)
  5. Sequential Function Charts (SFC)

These are the topmost 5 different type of PLC programming languages.
According to the study and operations, ladder diagram (LD) is the widely PLC language for writing easily understandable programming logic. This programming logic based on the logic gates.

Ladder Diagram Programming Example:

Representation Of Field/Electrical & PLC Contacts:

Behaviour of Contacts:

Same Contact= Resultant NO or XIC
Opposite Contact= Resultant NC or XIO
To Start= Require Resultant NO Contact
To Stop= Require Resultant NC Contact

PLC Sinking and Sourcing:

“Sinking and Sourcing – describes a current signal flow relationship between field input and output devices in a control system and their power supply.

The term Sinking and sourcing refer to current that flows into and out of a device.

The type of current that is sinking or sourcing is called Conventional Corrent Flow.

Conventional Current always flows from a positive potential to a more negative potential.


The term sinking refers to a device or component that accepts, or absorbs, current.

Conventional current actually flows into this sinking device.


The term Sourcing refers to a device or component that provides current.

Conventional current flows into this Source device.

Programmable Logic Controllers (PLCs) operate by receiving a variety of information. One type of information is an input Signal from from a sensor that functions as the PLC’S “eyes”.

When a sensor detects something, the sensor switches on the tell the PLC that it has sensed the presence of an object.

An input module is used by the PLC to perform the following functions.

  1. Provide a terminal to which the sensor (called a field device) can be electrically connected.
  2. Provide interfacing between the field device and the processor (brain) of the PLC. This is necessary because the sensor voltage and processor voltage are not the same.
  3. Turn on a light indicator that shows if the sensor is detecting an object.

There are two types of input modules, sinking and sourcing.

Sinking Module:

A Sinking input module is shown. when the sensor detects an object, it switches on and passes current.

Conventional current flows from the positive (+) terminal of a power supply, through the sensor, into the sinking module, and back to the negative terminal of the power supply.

Sourcing Module:

A Sourcing input module is shown. when the sensor detects an object, it switches on and passes current.

Conventional current flows from the positive (+) terminal of a power supply, through the input module, out of its sourcing input lead, through the sensor and back to the negative (-) terminal of the power supply.

Because the conventional current flows into the sensor, it is refferred to as a sinking device.

Therefore, a sinking type of sensor is connected to a sourcing input module.


The capacity to switch from primary equipment to standby equipment automatically without affecting the process under control. Redundancy means provision for standby module. In case of failure of one module is running process, the standby module takes over. Hot redundancy means the changeover of control from active processor to standby processor in less than 1 scan time.

In critical processes, it is important to run the plant without failure. In such case it is important to have redundancy so that even if one system fails the redundant system can take care without affecting plant.

Types of redundancy:

CPU redundancy: In case of CPU failure the standby CPU takes care of the plant.

Power Supply redundancy: In case the power supply fails the standby power supply takes control of the situation.

Communication: Multiple communication channels are provided to take care of communication failure.

I/O Redundancy: Multiple I/O channels are provided to take care of input or output failure.