IBM Monochrome Display Adapter
| IBM Monochrome Display and Printer Adapter | |
|---|---|
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| IBM Monochrome Display and Printer Adapter | |
| Manufacturer | IBM |
| Type | Text-only video adapter with parallel printer port |
| Release date | 1981 (with the IBM PC) |
| Interface | 8-bit IBM PC I/O channel |
| Compatible | IBM PC (5150), IBM PC XT (5160), IBM PC AT (5170) |
| Connectivity | 9-pin D-shell TTL monochrome video (for the IBM 5151); 25-pin D-shell parallel printer port |
The IBM Monochrome Display and Printer Adapter, usually called the MDA, is IBM's text-only video card for the IBM PC (5150). It shows 80 characters by 25 rows, each character in a 9 by 14 dot box, which gives a 720 by 350 dot screen, and it has no graphics mode. The same card carries a parallel printer port, so removing the MDA from a machine also removes that printer port.[1][2]
Hardware
[edit | edit source]IBM's technical reference describes the card as follows:[1]
- A Motorola 6845 CRT controller.
- 4 KB of display RAM with no parity, which the processor can access directly. Two bytes are fetched from it every 553 ns, a data rate of 1.8 MB/s.
- An 8 KB character generator holding 256 characters, each 7 by 9 dots in a 9 by 14 box.
- Output for an 18 kHz monitor. With a P39-phosphor display, which is what the IBM 5151 has, the card does not support a light pen.
- A printer section with 12 latched TTL outputs and 5 steady-state inputs.
The display buffer is at B0000h and the video registers occupy I/O addresses 3B0h to 3BFh. The 6845 is soldered in; the character generator is a socketed 9264 ROM, which has a different pinout from the common 2764.[2] The printer port is fixed at I/O address 3BC and uses interrupt 7.[3]
The card's 9-pin connector has ground on pins 1 and 2, +Intensity on pin 6, +Video on pin 7, +Horizontal on pin 8 and −Vertical on pin 9, at 0.0 to 0.6 V dc for low and +2.4 to 3.5 V dc for high. Pins 3 to 5 are unused.[1] On the earliest cards pins 3, 4 and 5 are driven low and there is an extra Berg connector, J2, for a light pen; later cards leave those pins unconnected.[4]
Set-up
[edit | edit source]In a 5150 or 5160 with the MDA as the only video card, the two video switches on the motherboard, SW1-5 and SW1-6, must both be off.[3]
The card does not produce proper sync signals at power-on. It only does so once the BIOS power-on self test, or a diagnostic ROM, has programmed it.[3] IBM's reference says the first command to the card must write hex 01 to port 3B8h to set high-resolution mode; if that bit is not set, any processor access to the card stops the processor.[1][2]
Faults
[edit | edit source]| Symptom | Cause | Action |
|---|---|---|
| One long and two short beeps at power-on | The 5150 BIOS sounds this when the video or horizontal sync bit in the display adapter's status port does not change during the power-on test.[5] | Reseat the card and check that SW1-5 and SW1-6 are both off.[3] |
| No raster on the 5151, even with brightness fully up | The 5151 needs horizontal sync from a working MDA to produce any raster. The cause can be the monitor, the cable connection, a faulty MDA, or an MDA that was never initialised because the motherboard switches are set for another card or the motherboard is faulty.[6] | Check the switches and the connector, then try a known-good MDA. |
| Stable raster but no text | On a 5151 the raster shows that the MDA's sync is correct, so the fault is in the video signal: contrast set too low, a faulty MDA, the video wire (pin 7) in the cable or plug, or the 5151's video path. At least two people have reported a failed IC201 (SN7406A) in the 5151.[6] | Turn up the contrast, try a known-good MDA, then check pin 7 and the monitor. |
| Wrong or garbled characters | The font is held in the socketed 9264 character ROM.[2] | The ROM can be read in the spare ROM socket on a 5150 motherboard, which has the same pinout, and a 2764 EPROM can be wired to replace it.[2] |
Do not program the 6845 with timings other than MDA's while a 5151 is connected. The 5151 can be damaged by an improper horizontal sync signal, and software can cause one.[6]
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 IBM, IBM Monochrome Display and Printer Adapter, section of Technical Reference: Options and Adapters, Personal Computer Hardware Reference Library, part number 6361511. Monochrome display adapter function pp. 1-10, printer adapter function pp. 11-18. Hosted as File:IBM Options and Adapters - IBM Monochrome Display and Printer Adapter.pdf, described at IBM Options and Adapters - IBM Monochrome Display and Printer Adapter.
- ↑ 2.0 2.1 2.2 2.3 2.4 John Elliott, Monochrome Display Adapter: Notes, seasip.info. Supports the 720 by 350 resolution, the socketed 9264 character ROM and its pinout, the soldered 6845, memory and I/O addresses, attribute behaviour and the high-resolution bit.
- ↑ 3.0 3.1 3.2 3.3 IBM 5150/5160 cards, minuszerodegrees.net, section "IBM Monochrome Display and Printer Adapter (MDA)". Supports the motherboard switch settings, the absence of proper sync at power-on until POST programs the card, and the printer port address and interrupt.
- ↑ IBM 5150 – Early Versions, minuszerodegrees.net, section "Early Cards – Monochrome Display and Printer Adapter (MDA)".
- ↑ IBM, Technical Reference, Personal Computer Hardware Reference Library, Revised Edition (April 1984), Section 5 "System BIOS", listing p. 5-40, lines 818-846 (CRT_ERR, DX = 0102h: one long, two short). Hosted as File:IBM PC 5150 Technical Reference.pdf.
- ↑ 6.0 6.1 6.2 IBM 5151 – Known Problems/Issues, minuszerodegrees.net.
Related pages
[edit | edit source]- IBM 5151, the monitor the card was designed for
- IBM Color Graphics Adapter
- IBM PC (5150), IBM PC XT (5160), IBM PC AT (5170)
- IBM Options and Adapters - IBM Monochrome Display and Printer Adapter, IBM's technical reference for the card