What is Programmable Logic Controller “PLC”

What is the definition of “PLC”

PLC full name is “programmable logic controller” A promising technology with ease of operations and development of logic for automation of plants. The system supports various types of inputs and outputs. The logic developed is stored in software forms and very easy to change online. The input and output side connections are fixed and need not be changed as per the logic.
PLC Advantages: Compact, reliable, flexible, and easy maintenance.

PLC Disadvantage : Expert manpower required for handling, costly.

History of PLCs

The first Programmable Logic Controllers (PLC) were designed and developed by Modicon as a relay re-placer for GM and Landis.

  • These controllers eliminated the need for rewiring and adding additional hardware for each new configuration of logic.
  • The new system drastically increased the functionality of the controls while reducing the cabinet space that housed the logic.
  • The first PLC, model 084, was invented by Dick Morley in 1969
  • The first commercial successful PLC, the 184, was introduced in 1973 and was designed by Michael Greenberg.

How does a PLC work?

PLC is defined as a digital electronic device that uses programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting, and arithmetic to control various machines and processes. The PLC consists of:

  • Input modules PLC
  • Output modules PLC
  • Central processing unit (CPU)
  • Memory of PLC
  • Power supply of PLC
  • Programming devices of PLC
    The block schematic of the PLC is as shown picture

Input Module of PLC

This is an interface between the field devices and CPU. This module converts the input signal into logical signal with which the CPU works. The block schematic of the typical input module

Main functions of input modules are:

  •  Indicating the status of input conditions
  • Providing electrical isolations 
  • Contact debouncing
  • Noise elimination
  • Signal conditioning for compatibility

There are various types of input modules. Based on the direction of electrical like Discrete input modules , Analog input modules ,Operating voltage: etc

Output Module of PLC

The output model of PLC module interfaces the actions generated by the CPU to the output devices. The block schematic of the output module is shown in picture as above PLC Diagram.

Types of Output Modules are:
Discrete output modules: This module transmits actions of the CPU to the devices like relays, pilot lamps, solenoids contactors, etc. Options like relays, Transistor-to-Transistor
Logic (TTLs), or isolated outputs are available for the selection of output module. The typical wiring diagram for the output module is as shown in picture. These cards consume power from backplane power supply.
Analog output modules: The CPU generates the digital signal, which in turn gets converted to analog signal using digital to analog converter (DAC or D/A), which is an integral part of this module. This type of module outputs signals in the form of 4–20 mA, 1–5 V. The specifications of the module are similar to that of input card. The high density card option is available for input/ output (I/O) modules. In these cards typically, 32 input or outputs can be connected to the CPU. This reduces the space required for the housing of the card in the chassis.

Central Processing Unit of PLC

This is the heart of the PLC. It controls the operation of the PLC. It is responsible for solving the logic as well as performing timing, counting, arithmetic, PID, and other advanced functions. The CPU sequentially carries out the
operation called “scanning.” The scan consists of series of sequential operations, which include data input, program execution, data output, housekeeping, updating the programming devices, communication with other devices, and diagnostics. Once the CPU is configured and put in RUN mode, it continues to repeat these operations. The CPU performs its operation in three modes viz. Program, RUN, and
Remote Run. In case of program mode, a user can enter/modify the program. The program needs to be downloaded in the CPU. The processor executes the program in the RUN mode. The status of the CPU can be seen from the indicators
located on the front facia of CPU. The indicators are RUN, FAULT, I/O, MEM, and BAT. Under normal working of the PLC, the MEM, FAULT, I/O, and BAT indication remains
off. If the PLC is in run mode, RUN is on.

Input scan of PLC : In this cycle, the CPU scans the status of all the inputs and stores it in the memory as input status table.
Program scan of PLC: The CPU solves the logic a user has stored in the memory in this mode. The results of the logic solve are sent to the output status file.
Output scan of PLC: The results received at the program scan are updated and physically the status of the output is changed in this scan. Scan time is defined as the time required for the CPU to complete one cycle of operation viz input scan, program scan, and output scan. Typically, the scan time is 7 ms/K of memory. Hence, the scan time depends on the size of the program, number of I/Os, and the processor used. House keeping: The operations like memory management, updating processor status file, and other internal registers are carried out in this scan cycle.

Power supply of PLC

In PLC, two types of power supplies are used. The power supply used for powering the cards and installed in the rack is called as back plane power supply. This power supply is generally specified by the manufacturer depending on the racks and cards ordered by the customer. The power supply is not used for driving the load in the field. The loads like transmitter, solenoid coils, pilot lamps, etc. are driven by bulk power supply. This is external to the PLC system. Depending on the number of loads to be driven and consumption of each load, the rating of the power supply is calculated. Generally, this power supply is customer’s requirement and needs to be ordered separately.

What is PLC Programing device:-

For modification and or loading of new program, a programming device is used. Various programming devices are available like hand-held communicator (HHC), PC, and memory chip. The selection of these programming devices depends on the stage of the project. In case of a new development, a PC is preferred whereas for the minor modifications at site, HHC is recommended and if the program is tested and no
changes are expected, then memory chip is used.


How is a PLC Programmed :-

 The PLC program can be developed using various programming methods and as per IEC 61131 Part 3 refers to the standard for programming a standard was developed
to apply uniform structures for programming all PLC.

PLC programming can be done different methods

Ladder diagram: Ladder diagram programming is a widely used for programming in PLC and mostly accepted way of PLC programming. In this technique, the program is expressed in the form of a ladder. It consists of two vertical lines. The left line represents positive or active power supply and the right line represents negative or neutral supply. Between the supply rails, lines called rungs made of various I/O devices are included in PLC.

Function block diagram: The function block diagram (FBD) is a technique in which the instructions are clubbed in one or more blocks. These blocks can be combined for the purpose of development of logic and Functional block diagram method programming in PLC is less popular as compared to Ladder  diagram.

Sequential function chart:    In some plants simultaneous operations are expected to be performed by the PLC. This concurrent processing is achieved with a graphical technique known as sequential function charts (SFC).

Instruction list: This programming method is similar to assembly language programming used in microcontrollers and This programming method is rarely used for programming in PLC.

Structured text: Similar to C language, this programming technique is developed. The program is called main and is written between statements PROGRAM and END_PROGRAM .

What are the types of PLC :-

There are  two types of Programmable logic controller (PLC)

  • Modular PLC
  • Fixed PLC

Modular PLC–  The number of input/output can be changed as per the user needs .

The modular PLC is a type that allows multiple expansions of the PLC system through the use of modules, hence the term “modular”.

Modules give the programmable logic controller (PLC) additional features like increased number of I/O units, and they are usually easier to use because each component is independent of each other.

The power supply, communications module, Input/Output module are all separate to the actual microcontroller so you have to manually connect them to each other to create your PLC control system.

A type of modular PLC is the rack-mounted or rack mount PLC. In a rack mount PLC, the communications module of the PLC resides in the rack itself, so all connections are centralized.

Fixed PLC :-  Input /Out put of the device remains fixed nothing can be add external.


All-In-One PLC :-

All-in-One, compact PLC + HMI with built-in I/Os fills a niche in the programmable controller market serving OEMs who need space saving controllers that deliver the functionality to control complex machines, while creating a competitive advantage via a color-touch HMI panel.

PLC Application :- 

PLCs are used in various applications in industries such as the steel industry, automobile industry, chemical , oli and gas industry and the energy sector. The scope of PLCs dramatically increases based on the development of all the various technologies where it is applied.


 PLC supplier :- 

ABB , Honeywell , Yokogawa , GE , AMCI Industrial , Unitronics , Schneider Electric , Omron PLC , Mitsubishi , Allen Bradley PLC

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