9 PIN D-SUB FEMALE at the videocard
9 PIN D-SUB MALE at the monitor cable
The IBM Monochrome Display Adapter (MDA), introduced with the original IBM PC in August 1981, outputs a TTL monochrome signal through a 9-pin DE-9 female socket on the card. It produces a 720×350 pixel display at 18.432 kHz horizontal sync and 50 Hz vertical sync, driven by a Motorola MC6845 CRT controller. The Hercules Graphics Card uses the identical DE-9 connector and pin assignments as the MDA for full text-mode compatibility, whilst adding a 720×348 bit-mapped graphics mode; there is no separate Hercules electrical standard — it conforms entirely to the IBM MDA electrical specification.
| Pin | Name | Description |
|---|---|---|
| 1 | GND | Ground |
| 2 | GND | Ground |
| 3 | - | - |
| 4 | - | - |
| 5 | - | - |
| 6 | I | Intensity |
| 7 | M | Mono Video |
| 8 | H | Horizontal Sync |
| 9 | V | Vertical Sync |
Signals
- GND — Signal ground (pins 1 and 2 are both ground returns).
- - — Pins 3–5 are not connected on standard MDA and Hercules cards. Some early MDA boards have circuitry capable of outputting TTL R/G/B signals on these pins, but this is non-standard.
- I — Intensity (High Intensity). Combines with the Video signal to produce four display states: black (Video=0, Intensity=0), dim green (Video=0, Intensity=1, non-standard on most monitors), green (Video=1, Intensity=0), and bright green (Video=1, Intensity=1).
- M — Mono Video. TTL-level luminance signal for the monochrome phosphor.
- H — Horizontal Sync. TTL positive pulse at 18.432 kHz.
- V — Vertical Sync. TTL active-low pulse (−Vertical Sync) at 50 Hz.
Notes
The DE-9 female socket is fitted at the video card; the monitor cable terminates in a DE-9 male plug. Signal levels conform to TTL: logic low 0.0–0.6 Vdc, logic high +2.4–3.5 Vdc (per IBM Technical Reference Options and Adapters, April 1984). Pin 9 vertical sync is active-low. The pixel clock runs at 16.257 MHz, yielding 720×350 text resolution at 80×25 characters. The Hercules card is electrically and mechanically identical to MDA on this connector; its 720×348 graphics mode uses the same sync timings.