PLC-Course

Sprint 1 Cheat Sheet — Architecture & I/O

🧱 The Five Core Blocks

flowchart LR
    PS[⚡ Power Supply] --> CPU
    PS --> IO
    CPU[🧠 CPU<br/>+ Memory] <--> IO[🔌 I/O Modules]
    PG[💻 Programming<br/>Device] <-.-> CPU
    IO --> Field[🏭 Field Devices<br/>sensors / actuators]

    style CPU fill:#3b82f6,color:#fff
    style PS fill:#f59e0b,color:#fff
    style IO fill:#22c55e,color:#fff
    style PG fill:#a855f7,color:#fff

Block	Role
Power supply	Converts mains to clean low-voltage DC for the controller and field power
CPU + memory	Executes the program, holds the I/O image, manages comms
I/O modules	Translate field voltages/currents to digital values (and back)
Programming device	Used at commission and for diagnostics — usually a laptop
Field devices	The physical world: switches, sensors, motors, valves

🔄 The Scan Cycle

┌──────────────────────────────────────────────┐
│  1. READ INPUTS — copy every input to RAM    │
│  2. EXECUTE  — run program against input image│
│  3. UPDATE OUTPUTS — write output image to HW│
│  4. HOUSEKEEPING — comms, diagnostics, watchdog│
└──────────────────────────────────────────────┘

Scan time = time to complete one full loop, typically 1–50 ms. Critical insight: an input pulse shorter than the scan time can be missed.

🔌 I/O Types — Quick Reference

Type	Discrete (digital)	Analog
What	On/off, true/false	Continuous value
Examples	Limit switch, push button, photoelectric, contactor, solenoid valve	Thermocouple, RTD, pressure transducer, VFD speed reference
Signal	24 VDC, 120 VAC, dry contact	4–20 mA, 0–10 V, 0–5 V
Resolution	1 bit	12, 14, or 16 bits typically

⚡ Sourcing vs. Sinking (PNP vs. NPN)

	Sourcing (PNP)	Sinking (NPN)
Where current flows	Out of the sensor into the input	From the input into the sensor
Common in	Europe, modern Siemens	Asia, older Allen-Bradley
Wiring rule of thumb	Switch on the +24 V side	Switch on the 0 V side

⚠️ Mixing them up will silently invert your input or fry the module. Always match the sensor type to the input module.

🎯 Common Field Devices

Sensors (inputs):

• Limit switch — mechanical, robust, low-cost
• Inductive proximity — detects metal, no contact
• Capacitive proximity — detects almost anything (including liquids)
• Photoelectric — through-beam, retro-reflective, diffuse
• Encoder — rotary position (incremental or absolute)
• RTD — temperature, very accurate, slow
• Thermocouple — temperature, wide range, less accurate
• Pressure transducer — usually 4–20 mA loop-powered

Actuators (outputs):

• Relay output — flexible, slow (~10 ms), wears mechanically
• Transistor output — fast, DC only, high cycle life
• Triac output — AC loads, mid-speed
• Contactor — switches motor power
• Solenoid valve — pneumatic / hydraulic actuation
• VFD — variable speed motor control, takes 4–20 mA or 0–10 V

🚨 Top 3 Beginner Mistakes

1. Picking discrete I/O when you needed analog — “valve open/closed” might really mean “valve position 0–100%.”
2. Ignoring scan time — fast events (encoder pulses, button bounces) need either fast scan or specialized high-speed counter modules.
3. Mixing PNP and NPN sensors on the same input card — always check the module spec.

🔍 Manufacturer Translations

Concept	Siemens	Allen-Bradley	Mitsubishi
Discrete input	DI / E	I:1/0	X0
Discrete output	DO / A	O:2/0	Y0
Analog input	AI / PEW	I:3.0	D100
Internal flag	M / Merker	B3:0/0	M0

This site is open source. Improve this page.