A Programmable Logic Controller (PLC) or Programmable Controller is a small computer used for automation of real-world processes. PLCs controls machinery such as factory assembly lines and usually uses a microprocessor. The program can control complex sequencing and is stored in battery-backed memory and/or EEPROMs. The PLC is designed and packaged for extended temperature ranges, can be opperated in dirty or dusty conditions. They have immunity to electrical noise, and is mechanically more rugged and resistant to vibration and impact.

The programmable logic controller has become an invaluable tool in industry. Therefore, there is a large need for trained personnel who can program and integrate programmable logic controllers. If this is a feld that interest you, you will need basic knowledge several of the major brands of Programmable Logic Controllers, and then become fluent in the technical aspect of one particular PLC.

Modular PLCs have a limited number of input and output connections. However, expansions are available if there need for more I/O conections.

Rack-style PLCs have processor modules with separate I/O modules. These may occupy many racks. Often, they have thousands of discrete and analog inputs and outputs. A special high speed serial I/O link is often used so that racks can be remotely mounted from the processor. This alows for reduction in the wiring costs for large plants. Some PLCs can communicate over a wide range of media including RS-485, Coaxial, and even Ethernet for I/O control at high speed networks.

PLC industrial controllers can handle the latest and emerging technologies and techniques such as IEC 1131 programming, industrial automation controllers, embedded controllers, supervisory control and data acquisition, fuzzy logic, radio frequency, step, stage and state logic programming. Programmable Controllers are programmable via software interfaced via standard computer interfaces and proprietary languages and network options.

Digital signals yield an on or off signal, which the programmable logic controller sees as Boolean values. Analog signals may also be used to opperate devices such as volume controls. These analog signals are seen by the programmable logic controller as "floating point" values.

The PLC is actually a control device that consists of a programmable microprocessor. It is programmed using a specialized computer language. Logic controllers are usually programmed in any one of several languages, ranging from ladder logic to Basic or C. However, originally, they were programmed in ladder logic, which is similar to a schematic of relay logic.

To work with a PLC system, the users need to be able to control the system and assess the state of the system. This is accomplished by what is often referred to as the human-machine interface (HMI). In computer science, the human-computer interaction (how the user interface to a computer program) refers to the graphical, textual and auditory information the program presents to the user, and the control sequences such as keystrokes with the computer keyboard, movements of the computer mouse, and selections with the touchscreen that the user employs to control the program.