PLC · Lesson 1 of 10

What is a PLC?

A PLC (Programmable Logic Controller) is a small, rugged computer for factories: it handles heat, dust and electrical noise, reads buttons and sensors, and runs motors and valves according to a program you write. Here you will see why factories left relay circuits behind, and how a PLC "thinks" in a cycle called the scan.

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1From relays to the PLC

In the past, every machine was controlled by relay circuits, contactors and timers wired together. That circuit was the "program". To change something small (a 5-second delay, or one more condition) you had to unwire, rewire and add relays.

Relay panel (relay logic)

The program = the wiring itself
Every change = manual work and material
Every timer and auxiliary relay = a part in the panel that can fail
Troubleshooting with a multimeter, wire by wire

PLC

Buttons and sensors are wired to the inputs, contactors to the outputs, and that is it
All the logic is a program: change it from a laptop in minutes
Hundreds of timers, counters and auxiliary relays inside the program, without buying a single part
Watch the program while it runs (online) and find where the fault is

Example: a filling machine had 30 relays and 8 timers. After the PLC: the same buttons, sensors and contactors, but all the auxiliary relays and timers became lines in the program. The panel became smaller and easier to maintain.

Note: a PLC does not replace contactors, overloads or breakers. The contactor still carries the motor current; the PLC only switches its coil. And emergency stop (E-Stop) circuits must still disconnect through hard wiring or a safety relay, not through the normal program alone.

2The scan cycle

A PLC does not watch the buttons "all the time" like a wire does. It runs 3 steps in a row and repeats them forever. Each round is called a scan:

Read inputs: it takes a snapshot of all terminals at once and stores it in the input image.
Execute: it runs through the program from start to end and calculates the results.
Write outputs: it sends the results to the output cards all at once.

A scan usually takes 1–20 milliseconds, depending on the program length. Try it yourself in slow motion:

The scan cycle in slow motion

Remember: the program works on the image taken at the start of the scan, and the outputs only change at the end. If a signal comes and goes in less than one scan, the PLC may never see it.

3Parts of a PLC

A PLC is either compact (everything in one box: CPU, inputs and outputs, like the small models) or modular (a rack where you mount the cards you need). Tap each card:

PLC rack

Card	What connects to it	Signal
DI digital input	Buttons, limit switches, proximity sensors, photocells, overload contacts	0 / 24V DC
DO digital output	Contactor coils, relays, pilot lamps, solenoid valves	Relay or transistor
AI analog input	Pressure / level / temperature / flow transmitters	4–20mA or 0–10V
AO analog output	Drive (VFD) speed, control valves	4–20mA or 0–10V

4Well-known brands

Brand	Examples	Programming software
Siemens	S7-1200, S7-1500, LOGO!	TIA Portal
Allen-Bradley (Rockwell)	Micro800, CompactLogix, ControlLogix	CCW, Studio 5000
Schneider	Modicon M221, M241, M340	EcoStruxure Machine Expert
Mitsubishi	FX5U, iQ-R	GX Works
Delta	DVP, AS series	ISPSoft
Omron	CP1E, NX1P	CX-Programmer, Sysmac Studio

All of them support Ladder (LD), plus other languages of the IEC 61131-3 standard: FBD (blocks), ST (structured text) and SFC (steps). We focus on Ladder because it looks like the contactor circuits electricians already know.

About the simulator: the simulator you will use from lesson 3 is a generic trainer. The logic is the same in every brand; only the look and the addresses differ a little.

5Common faults on site

Common faults on site:

The PLC is in STOP after someone downloaded a program or after a fault: all outputs are off. Check the RUN/STOP LED before anything else.
The ERROR / SF LED is on: a card is not seated correctly, a card is missing compared with the configuration in the program, or there is a power fault.
The 24V supply has dropped: a short in one sensor wire pulls down the whole supply, so all inputs go off at once.
The memory battery is empty on older models: the program or the values are lost when the power goes off. Always keep a copy of the program.

?Lesson questions

Answer at least 5 of 6 correctly to mark the lesson done.

The numbers (scan time, number of cards) are typical examples and vary by model. The simulator here is a generic trainer with IEC / Siemens style addresses: the exact addresses and instructions differ from one brand to another, and the manual of your PLC is the reference. Your progress is saved on this device only.
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