Video Signal Conversion Systems —CGA/EGA to VGA, RGB to VGA Guide
Keywords: CGA to VGA converter, EGA to VGA converter, RGB to VGA, video signal conversion system
1. Overview of Signal Conversion Scenarios
Industrial video signal conversion is essential when upgrading legacy CNC displays to modern LCD monitors. This guide covers the three most common conversion scenarios:
| Conversion Type | Input Signal | Output Signal | Primary Challenge |
|---|---|---|---|
| CGA to VGA | Digital TTL 15kHz | Analog RGBHV 31kHz | Digital-to-analog + scan rate doubling |
| EGA to VGA | Digital TTL 22kHz | Analog RGBHV 31kHz | Digital-to-analog + scan rate conversion |
| RGB to VGA | Analog RGBS/RGBHV | Analog RGBHV (standard VGA timing) | Sync separation + timing normalization |
2. CGA to VGA Conversion
2.1 Technical Details
CGA (Color Graphics Adapter) was introduced by IBM in 1981 and outputs 4-bit digital TTL signals at 15.75kHz horizontal frequency. To display CGA output on a modern VGA monitor:
- DAC conversion: Translate 4-bit TTL digital signals (0V/5V) to analog RGB signals (0-0.7V)
- Line doubling: Double the horizontal scan rate from 15.75kHz to 31.5kHz (VGA standard)
- Frame buffer scaling: Scale 320x200 or 640x200 resolution to 640x480 or 800x600
- Sync generation: Generate standard VGA H-Sync and V-Sync signals
2.2 Implementation
Kongto Technology JTX-CVR-001 converter handles all four steps automatically. Installation requires only connecting the CGA output cable to the converter input and the VGA output to the LCD display.
3. EGA to VGA Conversion
3.1 Technical Details
EGA (Enhanced Graphics Adapter) outputs 6-bit digital TTL signals at 21.85kHz. EGA conversion is similar to CGA but with:
- 6-bit color depth (64 colors vs. CGA's 16 colors)
- Higher horizontal frequency (21.85kHz vs. 15.75kHz)
- 640x350 native resolution
4. RGB to VGA Conversion
4.1 RGBS vs RGBHV
Many FANUC systems output analog RGB signals with composite sync (RGBS) rather than separate H/V sync (RGBHV). VGA monitors require separate sync signals. The conversion process:
- Sync separation: Extract horizontal and vertical sync pulses from the composite sync signal using a dedicated sync separator IC
- Signal conditioning: Clean up sync edges and adjust pulse widths to VGA specifications
- Impedance matching: Ensure 75Ω source impedance on all output lines
5. CGA DB9 Pinout Table
The standard CGA connector uses a 9-pin D-sub (DB9) female connector on the display adapter. Below is the complete pinout:
| Pin | Signal Name | Description | Voltage Level |
|---|---|---|---|
| 1 | GND | Ground | 0V |
| 2 | GND | Ground | 0V |
| 3 | Red | Red video signal (digital TTL) | 0V / 5V TTL |
| 4 | Green | Green video signal (digital TTL) | 0V / 5V TTL |
| 5 | Blue | Blue video signal (digital TTL) | 0V / 5V TTL |
| 6 | Intensity | Intensity / brightness control bit | 0V / 5V TTL |
| 7 | Reserved | Reserved (not used on standard CGA) | N/A |
| 8 | H-Sync | Horizontal sync signal at 15.75kHz | 0V / 5V TTL |
| 9 | V-Sync | Vertical sync signal at 60Hz | 0V / 5V TTL |
6. EGA DB9 Pinout Table
The EGA connector also uses a 9-pin D-sub (DB9) but with a different signal assignment than CGA:
| Pin | Signal Name | Description |
|---|---|---|
| 1 | GND | Ground |
| 2 | Red (high) | Red video signal, high-intensity bit |
| 3 | Red (low) | Red video signal, low-intensity bit (primary red) |
| 4 | Green (high) | Green video signal, high-intensity bit |
| 5 | Green (low) | Green video signal, low-intensity bit (primary green) |
| 6 | Blue (high) | Blue video signal, high-intensity bit |
| 7 | Blue (low) | Blue video signal, low-intensity bit (primary blue) |
| 8 | H-Sync | Horizontal sync signal at 21.85kHz |
| 9 | V-Sync | Vertical sync signal at 60Hz |
Note: EGA uses 2 bits per color channel (high/low) enabling 64 total colors (6-bit color depth), unlike CGA which uses a single bit per channel plus an intensity bit for 16 colors.
7. Common Troubleshooting Table
When installing CGA/EGA-to-VGA converters, the following issues are frequently encountered:
| Symptom | Cause | Solution |
|---|---|---|
| No display / blank screen | Incorrect input signal type selected; converter not receiving power; loose DB9 connection | Verify DIP switch settings match your CGA/EGA standard; check 12-36VDC power input; reseat DB9 connector and tighten screws |
| Flickering image | Horizontal frequency mismatch; inadequate power supply ripple rejection | Confirm the converter's auto-sync range covers your monitor's scan rate; use a regulated power supply (ripple < 50mV) |
| Wrong colors displayed | TTL signal polarity reversed; incorrect intensity bit mapping; resistor ladder DAC mismatch | Check CGA/EGA jumper settings (some boards swap intensity and blue); verify 74LS series driver ICs are functioning |
| Image too dark or too bright | Analog RGB voltage levels out of range; contrast trimmer misadjusted | Use a trimmer potentiometer on the converter board to adjust RGB gain; typical analog RGB should be 0.7V p-p at 75Ω |
| No sync / rolling image | Composite sync not separated correctly; sync polarity incorrect | Verify sync separator IC (e.g., LM1881) is receiving clean input; check that H-Sync and V-Sync are not swapped |
| Ghosting / blurry text | Poor shielding on input cable; excessively long cable run | Replace with shielded twisted-pair cable; keep cable length under 1.5 meters for TTL signals |
8. Cable Specifications Table
Proper cabling is critical for reliable signal transmission. Use the following specifications when building or selecting converter cables:
| Signal Type | Recommended Cable Type | Max Length | Shielding Requirements |
|---|---|---|---|
| CGA TTL (Digital RGBI) | 9-conductor shielded cable, 24 AWG stranded | 1.5 meters (5 ft) | Overall braid shield ≥ 85% coverage; drain wire connected to GND at source end |
| EGA TTL (Digital 6-bit RGB) | 9-conductor shielded cable, 24 AWG stranded | 1.5 meters (5 ft) | Overall foil + braid shield; individual twisted pairs for each color channel recommended |
| Analog RGBS | 75Ω coaxial (RG-59 or equivalent) for each RGB channel + separate sync wire | 5 meters (16 ft) | Individual coaxial shields per channel + overall jacket shield |
| Analog RGBHV (VGA) | Standard VGA cable (3+4 coax + control wires) | 10 meters (33 ft) | Double shielding (foil + braid) on each coax; ferrite core on both ends for runs > 5m |
| DB9 Breakout / Adapter | IDC ribbon cable with DB9 hood or solder-cup DB9 connectors | 0.3 meters (1 ft) | Not required for short adapters; use twisted-pair ribbon for runs > 0.3m |
| Power (12-36 VDC) | 2-conductor 18 AWG stranded, polarized connector | 3 meters (10 ft) | No shielding required; use twisted pair to reduce EMI coupling |
9. Installation Steps
Follow these steps to properly install your CGA/EGA-to-VGA converter system:
- Identify your source signal type —Determine whether your machine outputs CGA (15.75kHz, 4-bit TTL), EGA (21.85kHz, 6-bit TTL), or analog RGBS. Check the machine's service manual or inspect the video connector pinout. Most FANUC 6/11/15 series use CGA; FANUC 0/16/18/21 series may use analog RGBS.
- Power down all equipment —Turn off and disconnect power to both the CNC machine and the replacement LCD monitor. Never connect or disconnect video cables while equipment is powered on, as this can damage the TTL driver ICs.
- Connect the converter input —Attach the DB9 male connector from the machine's video output to the DB9 female input on the converter board. Tighten the thumbscrews firmly (0.3-0.5 Nm torque). If your machine uses a different connector (e.g., 6-pin Molex), use the appropriate adapter cable.
- Connect the converter output —Plug a standard VGA cable (male-to-male) from the converter's VGA output port to the LCD monitor's VGA input. Ensure the cable is fully seated and the thumbscrews are tightened. If the LCD monitor only accepts DVI-D or HDMI, use an active VGA-to-DVI/HDMI adapter.
- Apply power to the converter —Connect a 12-36VDC power supply to the converter's power input terminal. Observe correct polarity (center-positive for barrel jacks; check markings for screw terminals). The converter's LED indicator should light up within 2 seconds.
- Power on and verify operation —Power on the LCD monitor first, then the CNC machine. The converter should automatically detect the input signal timing and output a stable VGA image. If no image appears within 10 seconds, use the calibration buttons (if present) to cycle through preset modes, or check the troubleshooting table in Section 7.
10. Comparison of Conversion Solutions
| Parameter | JTX-CVR-001 (CGA) | JTX-CVR-002 (EGA) | JTX-CVR-003 (RGB) |
|---|---|---|---|
| Input Signal | CGA TTL 15kHz | EGA TTL 22kHz | RGBS analog |
| Output Signal | VGA 640x480@60Hz | VGA 640x480/800x600@60Hz | VGA 640x480/800x600@60Hz |
| Color Depth | 8-bit per channel | 8-bit per channel | 8-bit per channel |
| Operating Temp | -20°C to +70°C | -20°C to +70°C | -20°C to +70°C |
| Power | 12-36 VDC | 12-36 VDC | 12-36 VDC |
| MTBF | 50,000 hours | 50,000 hours | 50,000 hours |
| Price | CNY 850 | CNY 880 | CNY 750 |
Contact Kongto Technology at [email protected] for product inquiries.