What Is a PLC?

1. What Is a PLC?

PLC stands for programmable logic controller. It controls conveyors, pumps, valves, motors, sensors, robotic cells, and other industrial equipment.

Its main task is to read input signals, execute the programmed logic, and control outputs:

Sensors and buttons → PLC → motors, lamps, valves, relays
Button pressed → PLC reads the signal → PLC turns on an output → lamp lights up
Sensor detects a part → PLC starts the conveyor → part moves onward

A PLC does more than pass a signal through. It executes a program that defines the conditions under which equipment operates.

2. Where PLCs Are Used

PLCs are used throughout industrial automation:

• starting and stopping conveyors;
• controlling pumps, pneumatic cylinders, and valves;
• monitoring faults and controlling visual or audible alarms;
• operating packaging lines and communicating with robots;
• checking position sensors;
• implementing interlocks and operating permissives.

A conveyor program might check:

Is Start pressed?
Is Stop released?
Is the protective door closed?
Is there no active fault?
Is starting permitted?

Only when the required conditions are satisfied does the PLC activate the motor command.

3. Parts of a Simple PLC System

Inputs

Inputs are signals received from external devices: Start and Stop buttons, an emergency-stop button, a part sensor, a limit switch, a door sensor, or another device's ready signal.

Typical names are Start_Button, Stop_Button, Part_Sensor, Door_Closed, and Emergency_OK.

Outputs

Outputs control external devices: run a motor, open a valve, turn on a lamp, sound a buzzer, enable a drive, or start a robot.

Typical names are Motor_Run, Valve_Open, Lamp_On, Alarm_Buzzer, and Robot_Start.

Program

The program defines when outputs should turn on:

If Start is pressed and there is no fault, run the motor.
If Stop is pressed, stop the motor.
If the door is open, prevent starting.

In FBD, this logic is built from AND, OR, NOT, latches, timers, and other blocks.

4. Basic PLC Operation

A PLC operates cyclically. This repeated sequence is called the scan cycle:

- 1. Read inputs.
- 2. Execute the program.
- 3. Write outputs.

For example, it checks a button, evaluates the logic, and switches an output on or off. It then repeats the cycle. Executing the same program many times per second lets the controller respond to changing signals.

5. Example: A Button Controls a Lamp

There is one button and one lamp:

Button pressed → lamp on
Button released → lamp off

The PLC input receives the button signal; the output controls the lamp.

6. Signal Table

The output follows the input.

7. FBD Diagram

The simplest diagram connects an input directly to an output:

[Input_1] ───────────────> [Output_1]

This illustrates the basic path: input signal → program logic → output signal. Additional blocks can introduce conditions:

[Start_Button] ──┐
                 ├── [AND] ──> [Motor_Run]
[Emergency_OK] ──┘

8. Simulator Practice

Open the example in the PLC Portal editor and start the simulation.

• 1. Find Input_1 and set it to TRUE.
• 2. Check that Output_1 also becomes TRUE.
• 3. Return Input_1 to FALSE.
• 4. Check that Output_1 turns off.

The exercise shows the relationship between input, program, and output. On real equipment, Input_1 could be a physical button and Output_1 a lamp, relay, valve, or motor-start command.

9. What to Understand

An ordinary switch opens or closes an electrical circuit. A PLC receives a signal, processes it according to a program, makes a decision, and controls an output.

This supports more complex conditions, such as running a motor only when Start is pressed, Stop is released, the door is closed, no fault is active, and the line is ready. The PLC program describes that logic.

10. Key Points

A PLC controls equipment according to a program:

Inputs → PLC program → outputs

Inputs report external conditions. The program evaluates the required conditions. Outputs control actuators. FBD is a visual way to describe this logic using blocks and connections.