A Programmable Logic Controller, or PLC, is more or less a small computer with a built-in operating system (OS). This OS is highly specialized to handle incoming events in real time, i.e. at the time of their occurrence.
The PLC has input lines where sensors are connected to notify upon events (e.g. temperature above/below a certain level, liquid level reached, etc.), and output lines to signal any reaction to the incoming events (e.g. start an engine, open/close a valve, etc.).
The system is user programmable. It uses a language called "Relay Ladder" or RLL (Relay Ladder Logic). The name of this language implies that the control logic of the earlier days, which was built from relays, is being simulated.
There are some other languages also used 1. Sequential Function chart 2. Functional block diagram 3. structured Text 4. Instruction List
A PLC is primarily used to control machinery. Programs written for PLCs consists in simple terms on instructions to turn on and off outputs based on input conditions and the internal program. In this aspect, it is similar to a computer. However, one designed to be programmed once, and run repeatedly as needed. In fact, a crafty programmer could use a PLC to control not only simple devices such as a garage door opener, but their whole house, including switching lights on and off at certain times, monitoring a custom built security system, etc.
Most commonly, a PLC is found inside of a machine in an industrial environment. A PLC can run an automatic machine for years with little human intervention. They are designed to withstand most harsh environments.
The PLC is a purpose-built machine control computer
designed to read digital
and analog inputs from various sensors, execute a user defined logic program,
and write the resulting digital and analog output values to various output
elements like hydraulic and pneumatic actuators, indication lamps, solenoid
coils, etc.
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