Overview
The VGA connector is a 15-pin D-subminiature connector designed by IBM in 1987 for computer video output. Originally introduced with the IBM PS/2, it became the standard analog video interface on PCs, monitors, projectors, and HD televisions for decades.
Technical Specifications
- Connector type: DE-15 (also called HD-15 or DB-15(HD)), three-row 15-pin D-subminiature design
- Video signal: Analog RGBHV (red, green, blue, horizontal sync, vertical sync)
- Data channel: I²C-based DDC (Display Data Channel) for device identification
- Power output: Pin 9 provides +5V DC (50mA to 1A) per DDC specification
- Hot plugging: Not officially supported but typically functions without causing damage
Pin Configuration
| Pin | Signal | Function |
|---|
| 1 | RED | Red video signal |
| 2 | GREEN | Green video signal |
| 3 | BLUE | Blue video signal |
| 4 | ID2/RES | Reserved for E-DDC (formerly monitor ID bit 2) |
| 5 | GND | Ground for horizontal sync |
| 6 | RED_RTN | Red return signal |
| 7 | GREEN_RTN | Green return signal |
| 8 | BLUE_RTN | Blue return signal |
| 9 | KEY/PWR | +5V DC power for EDID chip |
| 10 | GND | Ground for vertical sync and DDC |
| 11 | ID0/RES | Reserved for E-DDC (formerly monitor ID bit 0) |
| 12 | ID1/SDA | I²C data line for DDC2 |
| 13 | HSync | Horizontal synchronization signal |
| 14 | VSync | Vertical synchronization signal |
| 15 | ID3/SCL | I²C clock line for DDC2 |
History and Development
- IBM introduced VGA with DE-15 connector in 1987 for PS/2 computers
- Early VGA clones used DE-9 (9-pin) connectors, omitting autodetection features
- DE-9 variants were compatible with each other; adapters to DE-15 were available
- All manufacturers eventually standardized on DE-15 connector
- VESA introduced DDC specification, reassigning monitor ID pins for serial bus communication
- Original IBM implementation used pins 9 and monitor ID pins differently than modern standard
- Modern digital interfaces (DVI, HDMI, DisplayPort) are gradually replacing VGA
Signal Characteristics and Capabilities
- VGA uses multiple scan rates; attached displays must be multisync-capable
- Original IBM implementation: two vertical refresh rates (60 and 70 Hz)
- Polarity of H and V sync signals communicated mode information to monitor
- Supports resolution range: 320×400 @70Hz (12.6 MHz bandwidth) to 2048×1536 @85Hz (388 MHz)
- Common resolutions include 1280×1024 (SXGA) @85Hz requiring 160 MHz bandwidth
- No formal quality standards exist for different resolutions
Cable Quality Considerations
- Higher-quality cables use coaxial wiring and enhanced insulation for thickness
- Good-quality cables prevent crosstalk (unwanted current induction between adjacent wires)
- Correctly impedance-matched cables (75 ohm) prevent signal ghosting
- Ghosting caused by impedance mismatches creating signal reflections
- Long cable ghosting may result from equipment termination issues or passive splitters
Alternative Connector Types
- BNC connectors: Some professional monitors use five separate 75-ohm coaxial BNC cables
- BNC provides superior quality with full coaxial shielding and reduced crosstalk
- BNC connectors are large, bulky, and require press-and-turn disconnection mechanism
- Mini-VGA: Two-row connector used in laptops and portable devices (early-to-mid 2000s)
- Mini-VGA significantly smaller than three-row DE-15 connector
Adapters and Conversion
- DVI-to-VGA adapters work because many DVI interfaces carry VGA-compatible analog signals
- HDMI and DisplayPort require active converters; passive adaptation is impossible (no analog output)
- Scan converters needed for digital format conversions; require external power and introduce signal loss
- VGA adaptable to SCART when correct sync rates and timing parameters are configured
- Software-adjustable graphics adapters can modify refresh rate, sync length, polarity, and blank lines
- VGA extenders (boosters) amplify signals for long cable runs or multi-monitor setups
Key Terms & Definitions
- DE-15: Accurate nomenclature for 15-pin "E" size D-sub connector
- DDC (Display Data Channel): Serial bus protocol for display capability advertisement
- EDID (Extended Display Identification Data): Digital data structure for display specifications
- RGBHV: Red, green, blue, horizontal sync, vertical sync analog video signals
- Crosstalk: Unwanted signal interference between adjacent conductors
- Multisync: Display capability to handle multiple scanning frequencies