1. Introduction to I/O Devices
Input devices send data into the computer.
Output devices present processed data from the computer to the user.
💡 I/O = Input/Output: Some devices (e.g., touchscreen, modem) perform both functions.
This study guide focuses on five essential I/O devices:
- Input: Keyboard, Scanner, OCR
- Output: Printer, Monitor
2. Input Devices
🔹 A. Keyboard
Function: Primary text and command input device.
Types of Keyboards:
| Type | Features |
|---|---|
| QWERTY | Standard layout (named after first 6 letters) |
| Mechanical | Individual switches under keys; durable, tactile feedback |
| Membrane | Rubber/silicone layers; quieter, cheaper |
| Wireless | Bluetooth/RF; no cables |
| Ergonomic | Split design to reduce strain |
| Virtual/On-screen | Touchscreen keyboards (smartphones, tablets) |
How It Works:
- Pressing a key closes a circuit → sends a scan code to the computer.
- OS converts scan code to character using keyboard layout (e.g., US, UK, AZERTY).
✅ Special Keys:
- Function Keys (F1–F12): Shortcuts (e.g., F1 = Help)
- Modifier Keys: Ctrl, Alt, Shift – modify other key functions
- Navigation Keys: Arrow keys, Home, End
🔹 B. Scanner
Function: Converts physical documents, photos, or objects into digital images.
Types of Scanners:
| Type | Use Case | Technology |
|---|---|---|
| Flatbed | Documents, photos, books | CCD (Charge-Coupled Device) sensor |
| Sheet-fed | Bulk document scanning | Paper pulled through scanner |
| Handheld | Small areas, portable use | Manual movement over document |
| Drum | High-resolution graphics (professional) | Photomultiplier tubes |
| 3D Scanner | Captures shape/texture of 3D objects | Laser or structured light |
Key Specifications:
- Resolution: Measured in DPI (Dots Per Inch) – higher = more detail (e.g., 600 DPI).
- Color Depth: Bits per pixel (e.g., 24-bit = 16.7 million colors).
- OCR Compatibility: Scanned images can be processed by OCR software to extract text.
🌐 Applications: Digitizing records, archiving, graphic design, passport control.
🔹 C. OCR (Optical Character Recognition)
Function: Software technology that converts scanned images of text into editable/searchable text.
How OCR Works:
- Preprocessing: Enhance image (deskew, noise removal).
- Character Recognition:
- Pattern Matching: Compares shapes to stored fonts.
- Feature Extraction: Analyzes lines, curves, loops (works with handwritten text).
- Post-processing: Correct errors using language dictionaries.
Popular OCR Tools:
- Tesseract (open-source)
- Adobe Acrobat (PDF text extraction)
- Google Drive (upload image → right-click → “Open with Google Docs”)
- ABBYY FineReader
Limitations:
- Poor image quality → low accuracy
- Handwriting recognition is less reliable than printed text
- Struggles with unusual fonts or layouts
✅ Real-World Use:
- Digitizing old books
- License plate recognition (ANPR)
- Bank check processing
- Accessibility tools for visually impaired (text-to-speech)
⚠️ Note: OCR is software, but it depends on input from scanners or cameras.
3. Output Devices
🔹 D. Printers
Function: Produce hard copies (physical output) of digital documents/images.
Types of Printers:
| Type | Technology | Pros | Cons | Best For |
|---|---|---|---|---|
| Inkjet | Sprays liquid ink through nozzles | High-quality photos, color printing, affordable | Slow, ink expensive, smudges | Home, photo printing |
| Laser | Uses toner (powder) + heat to fuse to paper | Fast, high-volume, sharp text, low cost/page | Expensive upfront, large size | Offices, bulk printing |
| Dot Matrix | Pins strike ribbon to create dots | Carbon copies, durable, low cost | Noisy, low quality, obsolete | Invoices, receipts (legacy systems) |
| Thermal | Heats special paper to create image | Quiet, no ink, compact | Fades over time, special paper needed | Receipts, labels (POS systems) |
| 3D Printer | Deposits material layer-by-layer | Creates physical 3D objects | Slow, expensive, complex | Prototyping, manufacturing |
Key Printer Terms:
- DPI: Print resolution (e.g., 1200 DPI = high detail)
- PPM: Pages Per Minute – speed metric
- Duplex: Automatic double-sided printing
🔹 E. Monitor (Visual Display Unit – VDU)
Function: Displays visual output (text, images, video) in real time.
Types of Monitors:
| Type | Technology | Features |
|---|---|---|
| CRT (Cathode Ray Tube) | Electron beam hits phosphor screen | Bulky, heavy, high power use (obsolete) |
| LCD (Liquid Crystal Display) | Liquid crystals block/allow light from backlight | Slim, energy-efficient, affordable |
| LED | LCD with LED backlight | Brighter, better contrast, thinner |
| OLED (Organic LED) | Each pixel emits its own light | Perfect blacks, high contrast, flexible screens |
| QLED | Quantum dot-enhanced LED | Wider color gamut, brighter than OLED |
Key Monitor Specifications:
- Resolution: Pixels (e.g., 1920×1080 = Full HD; 3840×2160 = 4K)
- Refresh Rate: Hz (e.g., 60Hz, 144Hz – higher = smoother motion)
- Response Time: ms (lower = less motion blur)
- Aspect Ratio: Width:Height (e.g., 16:9, 21:9)
💻 Touchscreen Monitors: Combine input + output (e.g., smartphones, tablets, kiosks).
4. Comparison Tables
Input Devices Comparison
| Device | Input Type | Output Format | Key Use |
|---|---|---|---|
| Keyboard | Keystrokes | Text/Commands | General computing |
| Scanner | Physical document | Digital image | Digitization |
| OCR | Scanned image | Editable text | Text extraction |
Output Devices Comparison
| Device | Output Type | Permanent? | Speed | Quality |
|---|---|---|---|---|
| Inkjet Printer | Paper | Yes | Slow | High (color) |
| Laser Printer | Paper | Yes | Fast | High (text) |
| Monitor | Screen | No | Instant | Depends on resolution |
5. Working Principles & Technologies – Summary
| Device | Core Technology |
|---|---|
| Keyboard | Electrical switch matrix → scan code |
| Scanner | CCD/CIS sensor + light reflection → digital image |
| OCR | Image processing + pattern/feature recognition → text |
| Inkjet Printer | Piezoelectric/thermal nozzles eject ink droplets |
| Laser Printer | Electrostatic drum + toner + fuser heat |
| Monitor | Pixel grid controlled by GPU; backlight (LCD/LED) or self-emissive (OLED) |
6. Applications & Use Cases
- Keyboard: Programming, typing, gaming, command-line interfaces.
- Scanner + OCR:
- Converting printed books to e-books
- Automating data entry from forms
- Assisting visually impaired users (scan → text-to-speech)
- Inkjet Printer: Home photo albums, school projects.
- Laser Printer: Office reports, legal documents, high-volume printing.
- Monitor: Video editing, gaming, web browsing, CAD design.
7. Glossary of Key Terms
- DPI: Dots Per Inch – measure of resolution (printers/scanners).
- CCD: Charge-Coupled Device – light-sensitive sensor in scanners.
- Toner: Powdered ink used in laser printers.
- Refresh Rate: How many times per second the screen updates (Hz).
- Scan Code: Numeric code sent by keyboard for each key press.
- Pattern Matching: OCR method comparing characters to stored templates.
- Duplex Printing: Automatic double-sided printing.
- Backlight: Light source behind LCD screens.