PLC Basics – Simplified and Clarified

plc basicsIf you are working or planning to work in a manufacturing or processing industry, then it is a good idea to get acquainted with the basics of PLC or Programmable Logic Controllers or Programmable Controller. A PLC is a major component in industrial automation and is an essential aspect of any automated manufacturing process.  The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. They were initially invented to replace the electromagnetic relays and the cumbersome wiring in a control circuit. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers.

The logic or sequence of operation for a process is executed as per a control logic program or software. A PLC is a microprocessor and a program is fed into a PLC so that it can control several functions in industrial processes. But it differs in some ways from a computer. Unlike desktop computers, PLCs have multiple inputs and outputs, and are designed for the rugged operation under extreme industrial conditions. They operate under extended temperature ranges, have immunity to electrical noise, and have resistance to vibration and impact.

Terms Frequently Used

Let us take a simple example to understand the different terms used in a circuit.

Consider the switching on and off of a light in a doorway and activation of a plunger to open the door when a switch is operated. In this system with a flick of the switch a light bulb would turn on or off.  But if you want the light bulb to switch off 30 seconds after you flip the switch to the ‘off’ position, you have to buy a timer and do some rewiring. But a PLC would save a lot of your time, labor and money to incorporate the above logic. Now consider the same device with a PLC in the middle.  The switch is fed as an input into the PLC and the light is controlled by a PLC output.  Implementing a delay in the switching off of the system is easy as the delay timer only needs to be incorporated in the PLC program. Also changing the time settings of the delay is very simple as it only requires you to change the timer value in the program. If you need to operate a plunger to open the door when the off switch is flicked, then you would need a relay to activate the coil of the plunger. But with a PLC you can do away with the relay and just need to wire the plunger coil to the PLC output. This is a rather simple example but in a larger system with many switches and control devices controlling several outputs, PLCs are imperative for control and automation.

Control Device: Any input device, for example an on/off switch, which controls the flow of current in a circuit. Control devices determine when loads should be energized or de-energized. They can be switches controlled by humans or can be contacts of relays.

Control Circuit: A type of circuit that shows the wiring of the control devices and the output loads. It shows how and when the current should be controlled to energize or de-energize the loads. Control circuits usually carry lower voltages than power circuits.

Relay: A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. They open and close circuits electromechanically or electronically. A relay can have two states, NO and NC. A relay contact is normally open (NO) when there is an open contact when the relay is not energized while in a Normally Closed (NC) state, there is a closed contact when the relay is not energized. In either case, applying electrical current to the coil of the relay will change the state of the contacts.

Relays are generally used to switch smaller currents in a control circuit and do not usually control power consuming devices except for small motors and Solenoids that draw low amps. But the relay contacts can be wired to switch large voltages of other electrical equipment. Protective relays are also available which prevent equipment damage by detecting electrical abnormalities, including over current, undercurrent, overloads and reverse currents. In addition, relays are also widely used to control the switching on and off of starting coils, heating elements, pilot lights and audible alarms.

Applications of PLC in Industry

A PLC may be seen as a digital industrial computer control system mainly used for automation of industrial processes, like controlling machinery or factory assembly lines. It continuously monitors the state of input devices, such as sensors, pressure switch, level switch, on/off switch, and makes decisions based upon a custom program or logic to control the state of the output devices like actuators, motors, relays or solenoids, and analog devices. It can also measure analog process variables, such as temperature and pressure and values from positioning and vision systems. The programs that are required to control the operation are usually stored in battery-backed or non-volatile memory.

PLC Plays Important Role in Automation

PLCs are indispensable to industrial automation. PLCs were invented as replacements for hundreds or thousands of relays, cam timers, and drum sequencers in a control circuit for automated systems. Often, a single PLC can be programmed to replace thousands of relays. The biggest advantage of programmable controllers are that they can replace the complications of rewiring the hard-wired control panels with simple software revisions when there is change in process, product model or sequence of operation. Expanding and contracting a process becomes much easier with a PLC based automation system.

Complicated wiring and need of hardware like timers, and relays can be reduced by using a PLC and writing software program to execute the control logic. This results in smaller and simpler control panels which occupy lesser space. Another advantage of a PLC system is that it is modular. That is, you can mix and match the types of Input and Output devices to suit your application.

Main Units and Structure of a PLC

A PLC consists of the following three main parts:

  • CPU (Central processing unit)
  • Input modules
  • Output modules
  • Power supply unit or module
  • Programming device

CPU: CPU or the Central Processing Unit is the heart of the PLC system. It executes the control instructions of the control logic program. It also communicates with other devices, which can include I/O Devices, Programming Devices, computers, Networks, and even other PLCs and also performs diagnostics tasks. The CPU reads input data from various sensing devices, executes the user program from memory and sends appropriate output commands to control devices.

Input modules: They are used for interfacing between input devices (such as start and stop push buttons, sensors, limited switch, selector switch) and microprocessor. The input devices which are in the field or remote locations are hard-wired to terminals on the input modules. Input modules accept signals from the machine or process devices and convert them to lower signals, such as 5VDC which are sent to the controller for processing. The input module also protects the processor from fluctuations in input signal voltages or currents.

Output modules: Output devices which are located in the field or remote locations such as small motor, motor starters, solenoid valve, and indicator lights are hard wired to the output modules.  The output interface modules convert the controller or processor signals into external signals which are used to control the output devices.

Power Supply Module:  This module provides the direct current (DC) power source required to produce low-level voltage used by processor and I/O modules. Depending on the PLC manufacturer, the power supply can either be housed in the CPU unit or may be mounted a separately mounted unit.

Programming device: This is a device, which is usually a personal computer, used to enter instructions or software program into the PLC. Sometimes they can be desktop computers or small hand held devices into which the PLC software program is loaded.

How to Program a PLC?

The earlier PLCs used simple relay ladder diagrams for setting up the logic and sequence. The electricians were able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. While Ladder Logic or Ladder Diagram is the most commonly used PLC programming language, it is not the only one. Some of the frequently used languages are Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC).


As you keep exploring PLCs for industrial automation applications you can get familiar with PLC units of different companies. The course Learn PLC in a Day – Wiring, Installation & Programming will introduce you to the PLC products of Allen Bradley, Delta, Siemens, and Schneider through a live interactive training system.