A PLC (Programmable Logic Controller) is a rugged computer that repeatedly reads what is happening in a machine, makes decisions using a stored control program, and commands the machine's devices. Unlike an office PC, it is built for electrical noise, vibration, heat and continuous operation.

Its normal scan cycle is:

1. Read inputs. The PLC copies the current state of every connected sensor into an internal input image.
2. Execute logic. It evaluates the program from top to bottom using that snapshot. The program may be ladder logic, structured text, a function-block diagram or a sequential-function chart.
3. Update outputs. It writes the calculated results to an output image and then energizes or changes the real outputs.
4. Housekeeping. It performs communications and diagnostics, then repeats the cycle—usually every few milliseconds.

Typical inputs include a pushbutton, limit switch, photoelectric sensor, pressure switch and emergency-stop status. Analog inputs represent a range rather than only on/off: for example, a 4–20 mA level transmitter or a 0–10 V temperature signal.

Typical outputs include a contactor coil, warning lamp and solenoid valve. Analog outputs can command a control valve position or the speed reference of a variable-frequency drive. Modern PLCs also exchange data with drives, remote I/O, HMIs and supervisory systems over industrial networks.

Simple example: automatically filling a tankSimple example: automatically filling a tank

• A low-level switch and a high-level switch are the inputs.
• A pump contactor is the output.
• When the low-level switch turns on, the PLC starts the pump.
• The pump remains on until the high-level switch turns on.
• An overload contact, emergency stop or maximum-run timer stops the pump and raises an alarm.

That last point is important: the program is not just sensor → motor. It also contains permissives, interlocks, alarms, manual/automatic modes and a defined safe state for faults.

PLCs are used because they provide predictable timing, electrically isolated industrial I/O, easy troubleshooting, modular expansion and maintainable logic. A technician can see which input, condition or interlock is preventing an output without rewiring the control panel. The PLC coordinates the process, but hardwired or safety-rated protection should still handle functions whose failure could injure someone.

A PLC (Programmable Logic Controller) is a rugged industrial computer that
repeatedly answers one question:

Given the machine’s inputs right now, what should its outputs be?

It is used instead of wiring every control decision permanently with relays.
The field wiring still carries the real signals, but the behavior can be
changed, diagnosed and expanded in software.

The three main partsThe three main parts

1. Inputs tell the PLC what is happening.
   Examples: pushbuttons, limit switches, photoelectric sensors, motor overload
   contacts, pressure switches, 4–20 mA temperature transmitters and encoder
   pulses.
2. The CPU and program apply rules to those input values.
   The program may use contacts/coils in Ladder Diagram, function blocks,
   Structured Text, timers, counters, arithmetic and state machines.
3. Outputs make something happen.
   Examples: contactor coils, solenoid valves, indicator lamps, alarms, analog
   speed references and commands sent to a VFD or robot.

What happens during one scanWhat happens during one scan

A normal PLC repeats a scan in milliseconds:

read inputs → execute program → update outputs → diagnostics/comms → repeat

For example, Siemens documents that its S7-1200 writes the output process
image, reads the physical inputs into an input process image, and then executes
the user program in order. Using an input image gives the program a consistent
snapshot during that scan:

https://cache.industry.siemens.com/dl/files/593/109741593/att_895681/v1/s71200_system_manual_en-US_en-US.pdf

Simple industrial example: filling a tankSimple industrial example: filling a tank

Inputs:

StartPB          operator requests automatic operation
LowLevel         tank needs more liquid
HighLevel        tank is full
MotorOverloadOK  pump protection has not tripped
EStopOK          safety circuit is healthy

Output:

PumpContactor    starts the filling pump

Plain-language logic:

Run the pump when automatic mode is requested and the level is low.
Keep it running until the high-level switch is reached.
Stop immediately on overload, emergency stop or sensor contradiction.

Equivalent simplified Structured Text:

IF NOT EStopOK OR NOT MotorOverloadOK OR (LowLevel AND HighLevel) THEN
    PumpRun := FALSE;
    Fault := TRUE;
ELSIF StartPB AND LowLevel THEN
    PumpRun := TRUE;
ELSIF HighLevel THEN
    PumpRun := FALSE;
END_IF;

PumpContactor := PumpRun AND NOT Fault;

The important engineering detail is that a PLC is not magic and software is not
the whole control system. Sensors must be selected and wired correctly, outputs
need suitable interposing relays/contactors, failures need defined safe states,
and emergency-stop functions normally require safety-rated hardware or a safety
PLC.

That combination—repeatable logic, industrial I/O, diagnostics and safe
interfacing—is why PLCs are used for conveyors, packaging lines, pumps, ovens,
compressors, traffic systems and process plants.

A PLC (programmable logic controller) is a rugged industrial computer whose main job is to make the same control decision, predictably, thousands of times a minute. A useful mental model is: sense -> decide -> act -> repeat.

During each scan the PLC normally:

1. Reads its physical inputs into an input image.
2. Executes the control program (ladder logic, structured text, function blocks, etc.).
3. Copies the calculated output image to the physical outputs.
4. Runs diagnostics and communications, then starts the next scan.

A scan often takes only a few milliseconds. Reading all inputs as a snapshot before solving the logic also makes the behavior easier to reason about than a general-purpose program reacting at arbitrary times.

Typical inputs include:

• Digital: a Start button, limit switch, photoelectric sensor, or overload contact (ON/OFF).
• Analog: temperature, pressure, flow, or level, commonly represented as 4-20 mA or 0-10 V.

Typical outputs include:

• Digital: a warning lamp, solenoid valve, contactor, or relay coil.
• Analog: a speed reference for a variable-frequency drive or a valve-position command.

Simple conveyor example: pressing Start does not directly power the motor. The PLC sees the Start input, checks interlocks such as "guard closed," "emergency stop healthy," and "motor not overloaded," then energizes the motor output. A photoeye at the end detects a box; the program can stop the belt, actuate a pneumatic pusher for one second, retract it, and restart the belt. Timers and sequence state make every cycle repeat consistently. If an interlock becomes false, the normal outputs drop to their defined safe state.

PLCs are used because their I/O is electrically isolated, their scan timing is deterministic, they tolerate heat/noise/vibration better than office computers, and technicians can troubleshoot live logic and I/O without rewriting an entire application. One important boundary: personnel-safety functions should use safety-rated relays or a safety PLC and an engineered safety circuit, not ordinary application logic alone.

A PLC (programmable logic controller) is a rugged industrial computer that repeatedly makes simple control decisions. It replaces a cabinet full of hard-wired relays with logic that can be tested, diagnosed and changed without rewiring the whole machine.

Its basic loop is called a scan:

1. Read inputs — copy the current state of sensors and switches into memory.
2. Run the program — evaluate the ladder logic, function blocks or structured text from top to bottom.
3. Update outputs — energize or de-energize output points according to the result.
4. Housekeeping — communications, diagnostics and fault checks; then the scan starts again, usually within milliseconds.

Typical inputs include a start button, emergency-stop status, limit switch, photo-eye, motor overload contact, tank float, temperature sensor or 4–20 mA pressure transmitter. Typical outputs include an indicator lamp, alarm horn, solenoid valve, contactor coil, heater relay, control valve command or VFD speed reference. The PLC normally commands a relay, contactor or drive—it does not feed a large motor directly.

Simple example: filling a tank. When the low-level switch is active and all safety permissives are healthy, the PLC starts the pump and opens the inlet valve. It keeps scanning the level switches. When the high-level switch turns on, it stops the pump and closes the valve. If the motor overload trips, flow does not appear within a timeout, or the level signals disagree, the PLC stops the process, latches a fault and turns on an alarm. An operator can see which condition failed instead of tracing dozens of relay wires.

PLCs are used because they tolerate electrical noise, heat and vibration; respond predictably; support industrial I/O and networks; and make machines easier to troubleshoot and maintain. The program is normally stored in non-volatile memory, while selected counters or settings can be configured as retentive so an ordinary power loss does not erase the machine logic.