IBM Color Graphics Adapter
| IBM Color Graphics Adapter | |
|---|---|
 |
|
| An IBM Color Graphics Adapter (CGA) card | |
| Manufacturer | IBM |
| Type | 8-bit ISA video display adapter |
| Release date | 1981 (with the original IBM PC) |
| Interface | 8-bit ISA |
| Compatible | IBM PC (5150), IBM PC XT (5160), IBM PC AT (5170), PC-compatible clones |
| Connectivity | DB-9 TTL RGBI (drives the IBM 5153); composite NTSC RCA; light-pen header |
The IBM Color Graphics Adapter (CGA) is the original 8-bit ISA colour graphics card IBM released alongside the IBM PC (5150) in 1981. It uses a Motorola MC6845 CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the IBM 5153) or a composite TV through an NTSC encoder.
Video Modes
| Mode | Resolution | Colours | Notes |
|---|---|---|---|
| 0 | 40 × 25 text | 16 fg / 8 bg | Colour burst on |
| 1 | 40 × 25 text | 16 fg / 8 bg | Colour burst suppressed |
| 2 | 80 × 25 text | 16 fg / 8 bg | |
| 3 | 80 × 25 text | 16 fg / 8 bg | Colour burst suppressed |
| 4 | 320 × 200 graphics | 4 from fixed palettes | Palette 0: cyan/magenta/white; Palette 1: green/red/yellow |
| 5 | 320 × 200 graphics | 4 (greyscale on composite) | Composite "alternate" palette |
| 6 | 640 × 200 graphics | 2 (background + foreground) | Highest standard resolution |
Several undocumented modes can be coaxed out of the 6845 (160 × 100 16-colour text-block mode, composite NTSC artefact colour, and various demoscene techniques), but they are not part of the BIOS interface.
Hardware
- MC6845 CRTC — clock and sync generation, addressing.
- 16 KB video RAM.
- TTL RGBI output on DB-9 (drives the IBM 5153).
- Composite NTSC output on RCA jack.
- Light-pen connector.
Common Faults
- Snow — the CGA "snow" effect is not a fault but a design quirk: the video RAM is single-ported and CPU writes during the active raster cause briefly-visible noise. Most CGA-aware games wait for the horizontal retrace to avoid it.
- No video, beep code 1 long + 2 short — reseat the CGA card first.
- Missing one colour channel — bad output buffer at the DB-9, or cracked solder joint on the connector.
- Smoke from a discoloured tantalum — replace the failed tantalum and any series resistor that may have died with it.