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Rebuilt from IBM's own Options and Adapters reference (now hosted here): correct card name, IBM's figures for the 4 KB unparity buffer, 553 ns fetch / 1.8 MB/s, 8 KB character generator, 9x14 box carrying a 7x9 character, no light pen with P39; register addresses added; fault list tightened and the 5151 sync-damage warning added; <ref>-cited
 
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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 [[IBM Color Graphics Adapter|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.
{{Infobox computer peripheral
| name          = IBM Monochrome Display and Printer Adapter
| image        = [[File:IBM MDA card.jpg|260px]]
| caption      = IBM Monochrome Display and Printer Adapter (MDA)
| manufacturer  = IBM
| type          = 8-bit expansion-slot video adapter (text only) with integrated parallel printer port
| release_date  = 1981 (with the original IBM PC)
| interface    = 8-bit IBM PC I/O channel
| compatible    = [[IBM PC (5150)]], [[IBM PC XT (5160)]], [[IBM PC AT (5170)]], PC-compatible clones
| connectivity  = 9-pin D-shell TTL monochrome (drives the [[IBM 5151]]); DB-25 Centronics parallel
}}
 
The '''IBM Monochrome Display and Printer Adapter''' &mdash; almost always shortened to '''MDA''' &mdash; is the text-only monochrome video card IBM released alongside the [[IBM PC (5150)]] in 1981. It has '''no graphics modes'''. What it does have is a large 9 by 14 character box, which gives a 720 by 350 dot image and makes text markedly crisper than [[IBM Color Graphics Adapter|CGA]] text at the same character count.
 
IBM's name for the card is not decorative: the same board also carries the PC's '''parallel printer port'''. Pulling the MDA out of a machine removes its printer port with it.<ref name="oamda">IBM, ''IBM Monochrome Display and Printer Adapter'', ''Options and Adapters'' section, Personal Computer Hardware Reference Library &mdash; hosted on this wiki as [[:File:IBM Options and Adapters - IBM Monochrome Display and Printer Adapter.pdf]]. Source for the Motorola 6845 CRT controller, the 4 KB display buffer with no parity, the two-bytes-in-553 ns fetch giving 1.8 MB/s, the 8 KB character generator holding 256 codes, the 9 PEL by 14 PEL character box carrying a 7 PEL by 9 PEL character, the 18 kHz monitor requirement, the statement that the adapter does not support a light pen with a P39-phosphor display, the register map, the 9-pin connector pin assignments and TTL signal levels, and the printer-adapter half with its 12 latched outputs and 5 steady-state inputs. Also documented here as [[IBM Options and Adapters - IBM Monochrome Display and Printer Adapter]].</ref>


== Hardware ==
== Hardware ==


* '''6845 CRTC''' — same timing chip as CGA, programmed for 18.432 kHz horizontal / 50 Hz vertical.
IBM's own description:<ref name="oamda" />
* '''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.
* '''Motorola 6845 CRT controller.'''
* '''TTL output''' on DB-9 (drives the [[IBM 5151]]).
* '''4 KB of display-buffer RAM''', dual-ported so the CPU has direct access, with '''no parity'''.
* '''Centronics parallel port''' on DB-25 — LPT1 by default.
* Two bytes are fetched from the display buffer every 553 ns, a data rate of 1.8 MB/s.
* '''8 KB character generator''' holding the fonts for 256 character codes.
* '''9 PEL by 14 PEL character box''', carrying a 7 PEL by 9 PEL character.
* '''18 kHz monitor''' &mdash; the card drives the [[IBM 5151]] and nothing slower.
* Per-character attributes.
* '''No light pen support''' when used with a P39-phosphor display, which is what the 5151 is.
* Printer half: 12 latched TTL output points, readable and writable under program control, plus 5 steady-state input points.
 
Where possible IBM held the card to a single low-power Schottky load per I/O slot line; no signal presents more than two.<ref name="oamda" />


== Video Mode ==
== Video Mode ==


* Mode 7 — 80 × 25 text, monochrome.
* '''Mode 7''' &mdash; 80 by 25 text, monochrome. There is no other mode.
* Per-character attribute byte selects: normal, bright, underline, reverse video, blink, invisible.
* The per-character attribute byte selects normal, bright, underline, reverse video, blink and invisible.
* No graphics mode.
* Registers: the 6845 index and data ports, the mode-control register at hex 3B8 and the status port at hex 3BA.<ref name="oamda" />


== Common Faults ==
== Common Faults ==


* '''No video, beep code 1 long + 2 short''' — the MDA card is dead or unseated. Reseat first.
* '''Card absent or not seated &mdash; one long beep, two short.''' That POST pattern means the system found no usable video adapter. Reseat the card before suspecting anything else.
* '''Garbled text''' — bad character ROM. Replace with a known-good donor.
* '''Garbled or wrong characters, correct layout.''' The 8 KB character generator ROM. Swap with a known-good donor.
* '''Missing or duplicated columns''' — bad VRAM chip. Swap one chip at a time with a known-good 4116.
* '''Missing or duplicated columns, or one character cell wrong everywhere.''' A failed display-buffer RAM chip.
* '''Smoke from a tantalum''' — replace the failed 22 µF / 16 V tantalum near the DB-9 output.
* '''Printer port dead, video fine.''' The failure is on the printer half of the card &mdash; the latches and buffers driving the DB-25 &mdash; and does not implicate the video path at all.
* '''Printer port dead but video fine''' — the 74LS373 latch or 74LS244 buffer on the parallel side has failed.
* '''A discoloured or smoking tantalum.''' The 51xx-family boards use 10 &micro;F 16 V tantalums for rail decoupling and they fail short. See [[IBM PC (5150) Capacitor Replacement Guide]].
 
'''A 5151 showing a stable raster and no characters is not necessarily a monitor fault''' &mdash; on a 5151 a stable raster proves the MDA is producing correct sync, so the video path is where to look, on either side of the cable.<ref name="mz5151">[https://www.minuszerodegrees.net/5151/problems/5151_known_problems_issues.htm IBM 5151 &mdash; Known Problems/Issues], minuszerodegrees.net. Source for the stiff brightness and contrast controls being normal, the raster-but-no-video diagnosis, the reported IC201 (SN7406A) failures, the vertical collapse and switch-off bright line notes, the statement that pins 3, 4 and 5 are absent from the connector by design, and the warning that an improper horizontal sync signal can damage the 5151.</ref>
 
'''Do not reprogram the 6845 outside MDA timings with a 5151 attached.''' An improper horizontal sync signal can damage that monitor.<ref name="mz5151" />


== Hercules Compatibility ==
== 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.
The Hercules Graphics Card (1982) added 720 by 348 graphics on top of MDA's text mode and the same 18.432 kHz timings, which is why it works with a 5151. '''A genuine IBM MDA has no graphics mode'''; software that expects Hercules graphics will not run on one.
 
== References ==
 
<references />


== Related Pages ==
== Related Pages ==


* [[IBM PC (5150)]]
* [[IBM 5151]] &mdash; the display this card was designed to drive
* [[IBM 5151]] — the matching monochrome monitor
* [[IBM Color Graphics Adapter]] &mdash; the colour alternative
* [[IBM Color Graphics Adapter]] — the colour alternative
* [[IBM PC (5150)]] &middot; [[IBM PC XT (5160)]] &middot; [[IBM PC AT (5170)]]
* [[IBM Options and Adapters - IBM Monochrome Display and Printer Adapter]] &mdash; IBM's own reference, hosted here
* [[IBM PC (5150) Capacitor Replacement Guide]]
* [[IBM PC (5150) Capacitor Replacement Guide]]
{{Navbox-IBMComputers|state=collapsed}}


[[Category:IBM]]
[[Category:IBM]]

Latest revision as of 03:20, 23 September 2026

IBM Monochrome Display and Printer Adapter
IBM Monochrome Display and Printer Adapter (MDA)
Manufacturer IBM
Type 8-bit expansion-slot video adapter (text only) with integrated parallel printer port
Release date 1981 (with the original IBM PC)
Interface 8-bit IBM PC I/O channel
Compatible IBM PC (5150), IBM PC XT (5160), IBM PC AT (5170), PC-compatible clones
Connectivity 9-pin D-shell TTL monochrome (drives the IBM 5151); DB-25 Centronics parallel

The IBM Monochrome Display and Printer Adapter — almost always shortened to MDA — is the text-only monochrome video card IBM released alongside the IBM PC (5150) in 1981. It has no graphics modes. What it does have is a large 9 by 14 character box, which gives a 720 by 350 dot image and makes text markedly crisper than CGA text at the same character count.

IBM's name for the card is not decorative: the same board also carries the PC's parallel printer port. Pulling the MDA out of a machine removes its printer port with it.[1]

Hardware

[edit | edit source]

IBM's own description:[1]

  • Motorola 6845 CRT controller.
  • 4 KB of display-buffer RAM, dual-ported so the CPU has direct access, with no parity.
  • Two bytes are fetched from the display buffer every 553 ns, a data rate of 1.8 MB/s.
  • 8 KB character generator holding the fonts for 256 character codes.
  • 9 PEL by 14 PEL character box, carrying a 7 PEL by 9 PEL character.
  • 18 kHz monitor — the card drives the IBM 5151 and nothing slower.
  • Per-character attributes.
  • No light pen support when used with a P39-phosphor display, which is what the 5151 is.
  • Printer half: 12 latched TTL output points, readable and writable under program control, plus 5 steady-state input points.

Where possible IBM held the card to a single low-power Schottky load per I/O slot line; no signal presents more than two.[1]

Video Mode

[edit | edit source]
  • Mode 7 — 80 by 25 text, monochrome. There is no other mode.
  • The per-character attribute byte selects normal, bright, underline, reverse video, blink and invisible.
  • Registers: the 6845 index and data ports, the mode-control register at hex 3B8 and the status port at hex 3BA.[1]

Common Faults

[edit | edit source]
  • Card absent or not seated — one long beep, two short. That POST pattern means the system found no usable video adapter. Reseat the card before suspecting anything else.
  • Garbled or wrong characters, correct layout. The 8 KB character generator ROM. Swap with a known-good donor.
  • Missing or duplicated columns, or one character cell wrong everywhere. A failed display-buffer RAM chip.
  • Printer port dead, video fine. The failure is on the printer half of the card — the latches and buffers driving the DB-25 — and does not implicate the video path at all.
  • A discoloured or smoking tantalum. The 51xx-family boards use 10 µF 16 V tantalums for rail decoupling and they fail short. See IBM PC (5150) Capacitor Replacement Guide.

A 5151 showing a stable raster and no characters is not necessarily a monitor fault — on a 5151 a stable raster proves the MDA is producing correct sync, so the video path is where to look, on either side of the cable.[2]

Do not reprogram the 6845 outside MDA timings with a 5151 attached. An improper horizontal sync signal can damage that monitor.[2]

Hercules Compatibility

[edit | edit source]

The Hercules Graphics Card (1982) added 720 by 348 graphics on top of MDA's text mode and the same 18.432 kHz timings, which is why it works with a 5151. A genuine IBM MDA has no graphics mode; software that expects Hercules graphics will not run on one.

References

[edit | edit source]
  1. ↑ 1.0 1.1 1.2 1.3 IBM, IBM Monochrome Display and Printer Adapter, Options and Adapters section, Personal Computer Hardware Reference Library — hosted on this wiki as File:IBM Options and Adapters - IBM Monochrome Display and Printer Adapter.pdf. Source for the Motorola 6845 CRT controller, the 4 KB display buffer with no parity, the two-bytes-in-553 ns fetch giving 1.8 MB/s, the 8 KB character generator holding 256 codes, the 9 PEL by 14 PEL character box carrying a 7 PEL by 9 PEL character, the 18 kHz monitor requirement, the statement that the adapter does not support a light pen with a P39-phosphor display, the register map, the 9-pin connector pin assignments and TTL signal levels, and the printer-adapter half with its 12 latched outputs and 5 steady-state inputs. Also documented here as IBM Options and Adapters - IBM Monochrome Display and Printer Adapter.
  2. ↑ 2.0 2.1 IBM 5151 — Known Problems/Issues, minuszerodegrees.net. Source for the stiff brightness and contrast controls being normal, the raster-but-no-video diagnosis, the reported IC201 (SN7406A) failures, the vertical collapse and switch-off bright line notes, the statement that pins 3, 4 and 5 are absent from the connector by design, and the warning that an improper horizontal sync signal can damage the 5151.
[edit | edit source]


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