What is a plc?

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What is a PLC?
The acronym PLC refers to (at least in the field of robotics) a Programmable Logic Controller. The automation of many electromechanical processes, such as the movement of machinery on an assembly line, is done through the use of small computers called programmable logic controllers (PLCs). A PLC contains a programmable microprocessor that is programmed using a specialized computer language. Typically, the program for the automated process is written on a computer and then is downloaded onto the programmable logic controller directly through a cable connection. The program is stored in the programmable logic controller in non-volatile memory.
PLCs are often used to control machines or processes that are sequential in nature, using “discrete” inputs and outputs that have defined states. For example, if a limit switch detects the presence of an object, it provides an “ON” signal to the PLC; if no object is detected, it provides an “OFF” signal. The machine or device typically
performs actions based on time or events in a pre-defined order. The expected sequence is typically interrupted only when an abnormal condition occurs.
inside of a plc
The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:

  • Execute the Control Instructions contained in the User's Programs. This program is stored in "nonvolatile" memory, meaning that the program will not be lost if power is removed

  • Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs.

  • Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.

Inputs and Outputs

Programmable logic controllers typically contain a variable number of input/output (I/O) ports and usually employ reduced instruction set computing (RISC), which consists of simplified instructions that are intended to allow for faster execution. PLCs are designed for real-time use and often must withstand harsh factory environments, such as excessive vibration and high noise levels. The programmable logic controller circuitry monitors the status of multiple sensor inputs, which control output actuators such as motor starters, solenoids, lights, displays and valves.


This type of controller has made a significant contribution to factory automation. Earlier automation systems had to use thousands of individual relays, timers and sequencers, which had to be replaced or rewired whenever the automated process needed to change. In many cases, a programmable logic controller allows all of the relays and timers within a factory system to be replaced by a single controller. Modern PLCs deliver a wide range of functionality, including basic relay control, motion control, process control and complex networking. They also can be used in a distributed control system (DCS).


There are several types of interfaces that are used when people need to interact with programmable logic controllers to configure them or work with them. The interface might be configured with simple lights or switches, or it might include a text display. A more complex system might use an Internet-based interface on a computer running a supervisory control and data acquisition (SCADA) system.


PLCs were first created to serve the automobile industry. The first programmable logic controller project was developed in 1968 for General Motors to replace hard-wired relay systems with electronic controllers. PLCs have remained widely used in the early 21st century within manufacturing sectors such as the automobile industry.

A Programmable Logic Controller is a device that a user can program to perform a series or sequence of events. These events are triggered by stimuli (called inputs) received at the programmable logic controller through delayed actions such as time delays or counted occurrences.

Once an event triggers, it actuates in the outside world by switching on or off electronic control gear or the physical actuation of devices. A Programmable Logic Controllers will continually loop through its user defined program waiting for inputs and giving outputs at the specific programmed times.

As you would imagine in the world of computers they have their own language. This language which is used to program the Programmable Logic Controller can be used in three formats, ladder, instruction list and logic symbol. More about this a bit later on.

Programmable Logic Controllers first came about as a replacement for automatic control systems that used tens and hundreds (maybe even thousands) of hard wired relays, motor driven cam timers and rotary sequencers. More often than not, a single PLC can be programmed to replace thousands of relays and timers. These Programmable Logic Controllers were first befriended by the automotive manufacturing industry, this enabled software revision to replace the laborious re-wiring of control panels when a new production model was introduced.

Many of the earliest Programmable Logic Controllers expressed all decision making logic in a program format called Ladder Logic, which from its appearance was very similar to electrical schematic diagrams. This of course was perfect for the electricians of the day, whom quite able to follow and trace out circuit problems with electrical schematic diagrams. So using ladder logic became second nature to them allowing the electricians an relatively easy transition from hard wired circuits to software driven circuits. This is the reason this program notation was chosen, to reduce training time for the existing technicians. Other early Programmable Logic Controllers used an instruction list type form of programming, based on a stack-based logic solver. Which was far most difficult to master.
So, what’s a Program?

I’m glad you asked!

A program is a connected series of instructions written in a language that the Programmable Logic Controller can understand. There are three forms of program format for PLC’s these are Ladder, Instruction and SFC/STL. Not all programming tools can work with all programming formats.
Generally hand held programming panels only work with instruction format while most graphic programming tools work with both instruction and ladder format. Specialist programming software will also allow SFC style programming but that’s for another time.

We will only be concerning ourselves with Ladder Logic programming here, because it's the most widespread in use today, probably because it's the easiest to grasp and get into the quickest.

Now, there's one big difference between a PLC and a PC type computer; as mentioned above, they only have one program to run. Unlike the PC, which is capable of running several programs at once within the Windows framework. Any of these could one or many, many more of the different programs that could be installed on the PC. Why? In one word, speed.
A PLC will be designed to run its one program at a very fast speed, only branching out from within the main bit when an event happens. Events that happen in real time. This gives our little PLC beastie the ability to respond very quickly to any of the events under its control via an input. Its response would then be carried out via an output. For example controlling a machines production running at 30,000 units an hour! Such as an offset web printing press churning out newspapers or book pages. Ladder Logic, (the PLC programming language) is very closely associated to relay logic. In relay logic there are both contacts and coils that can be loaded and driven in different configurations. As there are in ladder logic, but a lot more configurations are possible. However the basic principal remains the same. The program is written to switch the desired outputs for a given set of inputs energized. The 'hello world' program equivalent for a PLC would be a light bulb and a switch (see below). The switch is the input and the bulb would be controlled by the output. So, when the switch (input) is on, the bulb (output) is on.


A coil (relay logic terminology) drives outputs of the PLC (a ‘Y’ device, e.g. Y01) or drives internal coils (‘M’ device) timers, counters or flags. Each coil has associated contacts. These contacts are available in both normally open (NO) and normally closed (NC) configurations.

The term normally refers to the status of the contacts when the coil is not energized. Using a relay analogy, when the coil is off, a NO contact would have no current flow, that is, a load being supplied through a NO contact would not operate. However, a NC contact would allow current to flow, hence the connected load would be active. Activating the coil reverses the contact status, that is, the current would flow in a NO contact and a NC contact would then inhibit the flow.
Physical inputs to the PLC (X devices) have no programmable coil. These devices may only be used in a contact format, again with NO and NC types available. Because of the close relay logic association, ladder logic programs can be read as current flowing from the left vertical line to the right vertical line. This current must pass through the input (switch) configuration in order to switch the output coil Y0 on. Therefore in the example below, switching X0 on and X1 being off would causes the output Y0 to also to switch on. However, if X1 were to switch on while X0 was on, the output coil would then switch off.


This is a very basic example of course, as they are very capable of automating a complete warehouse or running very complex machines on their own. Then, as you would imagine, the program it would be running would have many twists and turns to respond to the 10's and quite possibly even 100's of inputs and outputs. These inputs in conjunction with the program would be dictating the on and off pattern of the outputs at any given time. Here are just a few examples of PLC programming applications that have been successfully completed and are in use today.

  • Manufacturing Industry
    - Lead acid battery plant, complete manufacturing system
    - Extruder factory, silo feeding control system

  • Travel Industry
    - Escalator operation, monitored safety control system
    - Lift operation, monitored safety control system

  • Aerospace
    - Water tank quenching system

  • Printing Industry
    - Offset web press print register control system
    - Multistage screen washing system

  • Food Industry
    - Filling machine control system
    - Main factory feed water pump duty changeover system

  • Textile Industry
    - Industrial batch washing machine control system
    - Closed loop textile shrinkage system

  • Hospitals
    - Coal fired boiler fan change-over system

  • Film Industry
    - Servo axis controlled camera positioning system

  • Corrugating
    - Main corrugation machine control system
    - BOBST platten press drive and control system

  • Plastics Industry
    - Extruder factory, silo feeding control system
    - Injection moulding control system

  • Agriculture
    - Glasshouse heating, ventilation & watering system

  • Foundry
    - Overhead transportation system from casting process to shotblasting machine

  • Leisure
    - Roller coaster ride and effects control system
    - Greyhound track 'Rabbit' drive system


One of the main benefits of PLC is perhaps the flexibility. A single machine holds the ability to perform multiple functions. It also holds the ability to control multiple programs in a single manipulation. The changes and errors in a PLC can be fixed by simply altering the circuit designs as well as the sequence. The maintenance and troubleshooting process of this machine is easy and trouble-free approach. Another prime advantage of a PLC is the fact that the size of the memory is big enough and multiple program systems can reside in the PLC itself. PLC boasts of immense durability and it can withstand extreme temperature conditions including moisture, heat and dust as well. The operator cost associated with Programmable Logic Controller is low and hence the productivity can be substantially increased as well.

One is sure to remain awe-stricken to come in terms with the different types of Programmable Logic Controllers available for sale. Programmable Logic Controllers were specifically designed for the purpose of automobiles and has gained paramount popularity in the equipment industry as well. Programmable Logic Controllers are widely used in the equipment industry due to its ability to withstand harsh environments. The services available in the marketplace include modification of PLCs tutorial and installation. Programmable Logic Controllers are expected to gain in popularity because of the convenience and cost-effective aspect.



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