PLC architecture, I/O modules and scan cycle

Why it matters

The programmable logic controller (PLC) is the workhorse of factory automation — conveyors, packaging machines, presses, pumps and process skids all run on one. Knowing what is inside it, how its I/O modules connect to real sensors and actuators, and how the scan cycle limits response time is essential for selecting hardware, wiring panels correctly and explaining why a fast event was "missed".

Key ideas

What a PLC is. An industrial computer built for harsh environments (electrical noise, vibration, 0–55 °C, 24 V DC or 230 V AC supply) that runs a user program cyclically and deterministically. It replaced hard-wired relay panels: logic changes are now software edits rather than rewiring.

Hardware architecture.

• Power supply: converts mains to the internal DC rails; often also gives 24 V DC for field sensors.
• CPU: executes the user program, manages communication and runs diagnostics, supervised by a watchdog that faults the PLC if a scan takes too long.
• Memory: firmware (ROM/flash), user program (flash), and data memory for the input/output image tables, internal bits (markers/flags), timers, counters and data registers. Retentive areas keep their values through power loss (battery-backed or non-volatile).
• I/O modules on a backplane (modular PLC) or built in (compact/brick PLC); remote I/O sits near the machine and connects over a fieldbus.
• Communication ports: programming port, Ethernet, serial, fieldbus.
• Programming device: a PC with IEC 61131-3 software (languages: Ladder Diagram, Function Block Diagram, Structured Text, Instruction List, Sequential Function Chart).

I/O modules.

• Discrete (digital) inputs: 24 V DC (most common) or 120/230 V AC, optically isolated, with a filter delay (typically a few ms, adjustable) that rejects contact bounce and noise. Sinking and sourcing describe the direction of current: a PNP (sourcing) sensor is wired to a sinking input module, an NPN (sinking) sensor to a sourcing input module.
• Discrete outputs: relay (AC or DC, a few A, slow and limited life — about 10⁵ to 10⁶ operations depending on load), transistor (DC only, fast, long life) and triac (AC only, silent).
• Analog inputs/outputs: 4–20 mA, 0–10 V, thermocouple and RTD modules, typically 12–16-bit, converting to an integer range defined by the manufacturer.
• Special modules: high-speed counters for encoders, positioning/motion, PID, weighing, and communication modules.

The scan cycle. The PLC operating system repeats:

1. Input scan: read all input modules and copy their states into the input image table.
2. Program execution: solve the program using the image tables — in ladder logic, rung by rung from top to bottom, each rung left to right — writing results to the output image table.
3. Output scan: copy the output image table to the output modules.
4. Housekeeping: communication, diagnostics, watchdog reset. Because inputs are frozen during execution, a single scan sees a consistent snapshot; but an input change is seen only at the next input scan, and an output changes only at the next output scan.

Timing consequences.

• Scan time is typically 1–50 ms, mainly set by program size and CPU speed.
• An input pulse shorter than one scan (plus filter time) may never be seen. Fast signals go to high-speed counter modules or interrupt inputs.
• Response time from input change to output change lies between about one scan and two scans, plus input-filter and output-device delays.
• Many PLCs offer immediate I/O instructions, periodic (time-driven) tasks and event tasks for faster or fixed-interval work.

Formulas

t_scan = t_in + t_prog + t_out + t_hk

• Scan time (s): input scan, program execution, output scan and housekeeping times.

t_prog ≈ N_instr × t_instr

• N_instr = instructions executed per scan; t_instr = average time per instruction (s).

t_resp,max ≈ t_filter + 2 × t_scan + t_out,dev and t_resp,min ≈ t_filter + t_scan + t_out,dev

• t_filter = input filter delay (s); t_out,dev = output device delay (relay pick-up, etc.) (s). The exact best case depends on where in the cycle the input changes; take these as the standard estimates.

t_pulse,min > t_scan + t_filter

• Minimum input ON (and OFF) time for reliable detection by normal scanning.

f_max ≈ 1 / (2 × (t_scan + t_filter))

• Highest pulse rate a normal input plus counter instruction can count reliably (equal on and off times) (Hz).

t_pulse = L / v

• Time an object of length L (m) blocks a sensor when moving at speed v (m/s).

Worked examples

Example 1 (standard). A PLC executes 20 000 instructions per scan at 0.4 µs each. Input scan 0.5 ms, output scan 0.5 ms, housekeeping 1 ms. The input filter is 3 ms and the relay output takes 10 ms to close. Find the scan time and the worst- and best-case response.

1. t_prog = 20 000 × 0.4 µs = 8 ms.
2. t_scan = 0.5 + 8 + 0.5 + 1 = 10 ms.
3. t_resp,max = 3 + 2 × 10 + 10 = 33 ms.
4. t_resp,min = 3 + 10 + 10 = 23 ms.

Answer: t_scan = 10 ms; response between about 23 ms and 33 ms.

Example 2 (GATE level). Bottles 60 mm in diameter, spaced 60 mm apart, pass a photo-sensor on a conveyor. The PLC scan time is 20 ms and the input filter 3 ms. (a) At 1.2 m/s, can a normal input count them? (b) At 3 m/s? (c) What is the maximum reliable count rate?

1. At 1.2 m/s: t_pulse = 0.060 / 1.2 = 50 ms (gap also 50 ms).
2. Requirement: t_scan + t_filter = 20 + 3 = 23 ms. 50 ms > 23 ms ✓ — counted reliably.
3. At 3 m/s: t_pulse = 0.060 / 3 = 20 ms < 23 ms ✗ — bottles can be missed.
4. f_max ≈ 1 / (2 × 0.023) = 21.7 Hz (bottles per second); at 3 m/s the rate is 3 / 0.12 = 25 bottles/s, which exceeds it.

Answers: (a) yes (50 ms ≫ 23 ms); (b) no — use a high-speed counter or interrupt input; (c) about 21.7 Hz.

Common mistakes

• Assuming outputs change the instant a rung becomes true — they change at the output scan.
• Forgetting that response time can approach two scans plus device delays.
• Counting fast pulses with ordinary inputs; use high-speed counter modules.
• Wiring a PNP sensor to an input module that needs an NPN (sinking) device.
• Using relay outputs for high-frequency switching (they wear out) or transistor outputs for AC loads.
• Ignoring input filter time in timing calculations.
• Writing the same output coil in several places in the program — only the last rung executed wins.

For GATE ME

PLCs appear in the mechatronics/automation portion as MCQs on architecture, input/output module types, sinking vs sourcing, IEC 61131-3 languages and the order of the scan cycle, and as short numericals on scan time, response time and whether a pulse of given length is detected. Practise adding times in consistent units and reasoning about where in the cycle an input changes.

Quick check

1. List the four phases of a PLC scan in order.
2. Which output type suits a 10 Hz solenoid on 24 V DC?
3. A PLC scans in 15 ms with a 5 ms input filter. Can it reliably see a 12 ms pulse?
4. What is the purpose of the watchdog timer in a PLC CPU?
5. Name the five IEC 61131-3 languages.

Answers: 1. Input scan, program execution, output scan, housekeeping 2. Transistor output 3. No (12 ms < 20 ms) 4. To fault the PLC safely if the scan exceeds its limit 5. LD, FBD, ST, IL, SFC