IBM Monochrome Display Adapter
| IBM Monochrome Display Adapter | |
|---|---|
 |
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| IBM PC Original Monochrome Display and Parallel Printer Adapter (MDA) | |
| Manufacturer | IBM |
| Type | 8-bit ISA video display adapter (text only) with integrated parallel printer port |
| 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 monochrome (drives the IBM 5151); DB-25 Centronics parallel (LPT1) |
The IBM Monochrome Display Adapter (MDA) is the original 8-bit ISA monochrome text adapter IBM released alongside the IBM PC (5150) in 1981. Unlike the CGA, the MDA has no graphics modes — it produces only 80 × 25 text in a 9 × 14 character cell — but the larger cell yields a 720 × 350-pixel image that is noticeably crisper for office work. The card also carries the IBM PC's parallel printer port on its back-panel DB-25 connector, an idiosyncratic combination that means removing the MDA also removes the printer port.
Hardware
- Motorola MC6845 CRTC — same timing chip as CGA, programmed for 18.432 kHz horizontal / 50 Hz vertical.
- 4 KB video RAM.
- Character ROM carrying the IBM MDA font in a 9 × 14 cell.
- TTL output on DB-9 (drives the IBM 5151).
- Centronics parallel port on DB-25 — LPT1 by default.
Video Mode
- Mode 7 — 80 × 25 text, monochrome.
- Per-character attribute byte selects: normal, bright, underline, reverse video, blink, invisible.
- No graphics mode.
Common Faults
- No video, beep code 1 long + 2 short — reseat the MDA card first.
- Garbled text — bad character ROM. Replace with a known-good donor.
- Missing or duplicated columns — bad VRAM chip.
- Smoke from a tantalum — replace the failed tantalum.
- Printer port dead but video fine — the 74-series latch or buffer on the parallel side has failed.
Hercules Compatibility
The Hercules Graphics Card from Hercules Computer Technology (1982) added 720 × 348 graphics on top of MDA's text mode. A genuine IBM MDA does not have these graphics modes — it is text-only.