What Is a PLC: CPU, Memory, I/O, the Scan Cycle, Scan Time, the IEC 61131 Standard and Choosing a PLC

Q: If a button is pressed for less than the scan time, will the PLC see it?

Possibly not. Inputs are read once at the start of each scan, so a pulse shorter than the scan time plus the input filter time can appear and disappear between two reads. Fast signals (encoders, high-speed part-counting sensors) need high-speed inputs, hardware counters or interrupts.

Q: Why does writing the same coil in two places make the program misbehave?

Because the PLC executes from top to bottom, the value written by the lower coil overwrites the value from the upper one before the outputs are sent at the end of the scan. The upper coil appears not to work (double-coil syndrome). Combine the conditions into a single rung, or use SET/RESET deliberately.

In 1968 General Motors' Hydra-Matic transmission plant wanted to stop rewiring relay panels every time the car models changed. That brief produced the first PLC (the Modicon 084). The core idea is unchanged today: a computer built to survive the factory floor, which reads inputs from switches and sensors, decides with a program an electrician can read, and drives outputs — repeating predictably, without end.

By Ajarn ModUpdated 14 Oct 2026

In 30 seconds

PLC = power supply + CPU + memory + I/O modules + communication ports
One scan: read inputs into the image → run the program top to bottom → write outputs → system tasks
Typical scan time 1–20 ms · worst-case response ≈ input delay + 2 scans + output delay
Its strength is robustness and deterministic behaviour, not speed

PLC architecture Basic

Figure 1 · The PLC sits between two groups of field devices: those that “tell” (buttons, sensors) and those that “do” (contactors, valves, lamps)

Part	Function	What to know
Power supply	Converts 230 VAC to 24 VDC for the CPU and I/O	Use a separate supply for high-current field loads (solenoids)
CPU	Runs the program, manages I/O and communication	Look at speed per instruction, memory size and special functions (PID, fast counters)
Memory	Stores the program, data and settings	Retentive data keeps its value through a power failure (battery/flash)
Digital I/O	0/1 signals from switches and to coils	24 VDC sinking/sourcing, relay/transistor outputs
Analog I/O	Continuous 4–20 mA, 0–10 V signals	12–16-bit resolution, must be scaled to engineering units
Communication	Talks to HMIs, SCADA, drives and other PLCs	Ethernet (PROFINET, EtherNet/IP, Modbus TCP), RS-485 (Modbus RTU)

The scan cycle Intermediate

A PLC does not “watch” its inputs continuously. At the start of each scan it takes a snapshot of every input terminal into the input image table, then runs the program rung by rung from top to bottom, reading from that snapshot. Coil results are written to the output image, which is sent to the output terminals all at once at the end of the scan. Working this way means input values cannot change part-way through a scan, so the result is predictable.

Figure 2 · Right: button I0.0 changes in the middle of scan 0, is read into the image at the start of scan 1, and output Q0.0 changes at the end of scan 1 (seen at the terminal in scan 2)

F1 Scan time

t_{scan} = t_{read\,I} + t_{program} + t_{write\,Q} + t_{comm/diag}

F2 Program execution estimate

t_{program} \approx N_{bit}\,t_{bit} + N_{word}\,t_{word} + N_{real}\,t_{real}

F3 System response time

t_{in} + t_{scan} + t_{out} \;\le\; t_{resp} \;\le\; t_{in} + 2\,t_{scan} + t_{out}

F4 Shortest pulse surely seen

t_{pulse} > t_{scan} + t_{in}

Example F-1

A conveyor runs at 1.2 m/s; 30 mm parts pass a photoelectric sensor; PLC scan 10 ms, input filter 3 ms

Time the part blocks the sensor: $t = 0.030/1.2 = 25$ ms
F4: must exceed $10 + 3 = 13$ ms → 25 ms is caught ✓ (almost a 2× margin)
If the conveyor speeds up to 3 m/s: $t = 10$ ms < 13 ms → parts may be missed; use a high-speed input or a hardware counter
F3: the time from a part reaching the sensor to the air-jet reject (transistor output 0.1 ms + solenoid 15 ms) is at worst ≈ 3 + 20 + 0.1 + 15 = 38 ms → the part travels 46 mm in that time, so mount the air nozzle this far beyond the sensor

Types of PLC Basic

Type	Characteristics	Good for
Nano/Compact (all-in-one)	CPU and I/O in one housing, 10–60 points, limited expansion	Single machines, pumps, building services
Modular (rack)	Choose the CPU, I/O and communication modules as needed, hundreds to thousands of points	Production lines, process plants
Safety PLC	Dual CPUs, self-checking, SIL/PL certified	Emergency stops, light curtains, safety gates, burner management
PAC / IPC (soft PLC)	An industrial computer running an IEC 61131-3 runtime	Heavy computation, multi-axis motion, large data volumes
Smart relay	Programmable relay with a small display and function blocks	Very small jobs, replacing a few relays

PLC compared with relays and microcontrollers

Criterion	Relay panel	PLC	Microcontroller
Changing the logic	Rewire	Edit the program	Edit the program (written in C)
Industrial robustness	High	High (IEC 61131-2 tests for EMC, temperature, vibration)	User must design it in
Diagnostics	Measure point by point	Online status view, event logs	Depends on the programmer
Predictability	High	High (fixed scan, watchdog)	Depends on the design
Cost per point	Low for very small jobs	Moderate	Very low

The IEC 61131 standard Intermediate

IEC 61131 makes PLC knowledge transferable across brands. The parts electricians and engineers use most:

Part 2 (equipment): 24 VDC input Types 1/2/3, ON/OFF voltage ranges, EMC tests — details in I/O wiring
Part 3 (languages): LD, FBD, ST, SFC (and IL, deprecated in the 2013 edition), data types, standard timers and counters — see IEC 61131-3 languages

How to choose a PLC Intermediate

F5 I/O count with spare

N_{I/O} = \left\lceil 1.2\,N_{required} \right\rceil \quad (\text{15–25 \% spare})

Make an I/O list: count digital and analog inputs and outputs and high-speed inputs, with their types (PNP/NPN, 4–20 mA)
The required response time (F3) and special functions (PID, counters, motion)
Communication with the HMIs, drives and SCADA systems already in the plant
Safety: is a safety PLC or safety relay needed?
Environment: panel temperature, humidity, vibration
Plant standards and spares on hand — a brand the maintenance staff already know is often better value than a cheaper one

Frequently asked questions

If a button is pressed for less than the scan time, will the PLC see it?

Possibly not. Inputs are read once at the start of each scan, so a pulse shorter than the scan time plus the input filter time can appear and disappear between two reads. Fast signals (encoders, high-speed part-counting sensors) need high-speed inputs, hardware counters or interrupts.

Why does writing the same coil in two places make the program misbehave?

Because the PLC executes from top to bottom, the value written by the lower coil overwrites the value from the upper one before the outputs are sent at the end of the scan. The upper coil appears not to work (double-coil syndrome). Combine the conditions into a single rung, or use SET/RESET deliberately.

Ajarn Mod · Teerawut Savangboon
Lecturer in Electrical Engineering, Dhonburi Rajabhat University, Samut Prakan · Profile