IBM Monochrome Display Adapter
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
- 6845 CRTC — same timing chip as CGA, programmed for 18.432 kHz horizontal / 50 Hz vertical.
- 4 KB video RAM — enough for one screen of text plus attribute bytes.
- 2 KB character ROM — the famous IBM "MDA font", a 9 × 14 cell with serifs at low pixel counts.
- 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 — the MDA card is dead or unseated. Reseat first.
- Garbled text — bad character ROM. Replace with a known-good donor.
- Missing or duplicated columns — bad VRAM chip. Swap one chip at a time with a known-good 4116.
- Smoke from a tantalum — replace the failed 22 µF / 16 V tantalum near the DB-9 output.
- Printer port dead but video fine — the 74LS373 latch or 74LS244 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 by adding 64 KB of additional video RAM and a second 6845 page. A genuine IBM MDA does not have these graphics modes.
Related Pages
- IBM PC (5150)
- IBM 5151 — the matching monochrome monitor
- IBM Color Graphics Adapter — the colour alternative
- IBM PC (5150) Capacitor Replacement Guide