What Is a PLC?
A programmable logic controller, or PLC, is a rugged industrial computer that helps control machines and processes. It receives information from devices in the field, follows a program, and sends commands to equipment.
You might find a PLC controlling a conveyor, a water treatment pump, a packaging machine, or a building's ventilation system. Each application is different, but the basic job is the same: monitor what is happening and respond according to the control program.
A simple example
Imagine a tank that needs to be filled with water. A level sensor tells the PLC when the tank is low, and a pump supplies water. The program uses the sensor information to decide when to command the pump on and when to stop it.
The PLC does not move the water itself. It receives a signal from the sensor and sends a command through an output to equipment that controls the pump. The pump, its power circuit, the sensor, and the PLC each have their own part to play.
Inputs, program, and outputs
Three pieces appear in almost every PLC control system:
| Part | Job | Examples |
| Inputs | Bring information into the PLC about the machine or process. | A pushbutton, a tank level sensor, or a temperature transmitter. |
| Program | Uses that information and the programmed rules to decide what should happen. | Start a pump when the level is low; stop it when the tank is full. |
| Outputs | Send commands from the PLC to devices that affect the process or inform an operator. | A pump starter, a valve, or an indicator light. |
Some signals have two states, such as a pushbutton that is pressed or released. Others represent a changing measurement, such as temperature or tank level. PLCs can work with both kinds.
What is inside a PLC system?
The processor, often called the central processing unit (CPU), runs the control program and manages information. Input and output modules connect the controller to field devices. A power supply provides the power the controller needs. Depending on the system, these parts may be combined in one compact unit or installed as separate modules.
A PLC can also communicate with other equipment. For example, an operator might use a touchscreen, called a human-machine interface or HMI, to see the tank level and request a pump start. The PLC and HMI have different roles: the HMI presents information and controls to a person; the PLC runs the control program.
How does a PLC keep a machine running?
A PLC repeatedly reads its inputs, runs its program, and updates its outputs. This repeating cycle is often called a scan. In our tank example, the PLC keeps checking the available input information and applying the pump rules as conditions change.
The exact timing and details depend on the controller and its configuration. For now, the key idea is that the PLC keeps checking and responding while it is operating. You will look at the scan more closely in a later lesson.
Why use a PLC?
Before programmable controllers, many control systems used banks of hardwired relays and timers. Changing how a machine worked could mean changing the wiring. A PLC can perform control decisions in a program, making many changes easier to develop, inspect, and maintain. It still needs proper field wiring, and a change to a program must be checked carefully before a machine is operated.
PLC hardware is designed for industrial environments and can be arranged to suit machines with different numbers and types of signals. Programs can be written in several languages; Ladder is one common choice, and its diagrams resemble aspects of relay circuits.
Some machines use an industrial personal computer (PC) running a software PLC instead of a separate dedicated PLC. Both can perform controller duties when properly configured. Learn more in What Is an Industrial PC?.
A useful distinction
A PLC's output command is not proof that the equipment actually responded. A program might command a pump to run while a disconnected wire, a tripped overload, or a failed pump prevents it from running. A separate feedback signal can help the controller or operator tell the difference between a command and the actual condition.
Some functions, such as emergency stopping, require a properly designed safety system. Do not assume that an ordinary PLC program alone provides that protection.
Key takeaways
A PLC is an industrial computer that runs a control program.
Inputs provide information; the program makes decisions; outputs send commands.
The controller repeatedly reads, executes, and updates as the process changes.
A PLC works with field devices and operator interfaces as part of a larger control system.
Next, explore the field devices that bring information into a PLC and act on its commands. When you are ready to learn how on/off signals are represented in a program, see Binary Signals and Why PLCs Think in TRUE/FALSE.
Enable JavaScript to run the embedded simulator and complete the interactive exercises.