9 PIN D-SUB FEMALE at the videocard
9 PIN D-SUB MALE at the monitor cable
The IBM Color/Graphics Monitor Adapter (CGA), introduced with the original IBM PC in August 1981, was IBM’s first colour display standard for the PC platform. It uses a 9-pin DE-9 connector to carry four-bit RGBI TTL digital colour signals, producing up to 16 colours from a fixed palette. The adapter supports 40×25 and 80×25 alphanumeric modes, as well as 320×200 four-colour and 640×200 two-colour graphics modes, at a horizontal sync rate of 15.7 kHz and a vertical rate of 60 Hz. A secondary RCA jack on the card provides a composite NTSC video output for use with television monitors or RF modulators.
| Pin | Name | Description |
|---|---|---|
| 1 | GND | Ground |
| 2 | GND | Ground |
| 3 | R | Red |
| 4 | G | Green |
| 5 | B | Blue |
| 6 | I | Intensity |
| 7 | RES | Reserved |
| 8 | HSYNC | Horizontal Sync |
| 9 | VSYNC | Vertical Sync |
Signals
- GND — Signal and chassis ground (pins 1 and 2).
- R — Red primary colour, TTL digital (pin 3).
- G — Green primary colour, TTL digital (pin 4).
- B — Blue primary colour, TTL digital (pin 5).
- I — Intensity bit; when asserted, brightens all three colour channels to produce the upper eight colours of the 16-colour RGBI palette (pin 6).
- RES — Reserved; not assigned a signal in the IBM specification and should be left unconnected (pin 7).
- HSYNC — Horizontal drive pulse, TTL positive, at 15.7 kHz (pin 8).
- VSYNC — Vertical drive pulse, TTL positive, at 60 Hz (pin 9).
Notes
- All signals are TTL-level: logic low 0.0–0.6 V, logic high +2.4–3.5 V, as specified by IBM.
- The card presents a DE-9 female socket; the monitor cable terminates in a DE-9 male plug.
- CGA does not have its own crystal oscillator; it derives its 14.318 MHz dot clock from the ISA bus.
- In addition to the DE-9 connector, the card carries: an RCA jack for composite video output; an internal 4-pin header (P1) for an RF modulator; a 6-pin light pen I/O header (P2); and a 2-pin font-select jumper (P3).
- The IBM PCjr provides CGA-compatible colour output but uses an 18-pin dual-row Berg-style header on the system board rather than a DE-9 socket; it is not pin-compatible with this connector.
- The adapter uses a Motorola MC6845 CRT controller.
References
- IBM Technical Reference: Options and Adapters Volume 2 (April 1984) — IBM Corporation (via Bitsavers / Internet Archive)
- IBM Personal Computer Technical Reference (August 1981) — document 6025008 — IBM Corporation (via Bitsavers / Internet Archive)
- Colour Graphics Adapter Notes — seasip.info — John Elliott