IBM Monochrome Display Adapter

IBM MDA Card with Hitachi HD6845 (= MC6845) and printer port.

IBM 5151 monitor driven by a Monochrome Display Adapter

The Monochrome Display Adapter (MDA, also MDA card, Monochrome Display and Printer Adapter, MDPA) introduced in 1981 was IBM's standard video display card and computer display standard for the PC. The MDA did not have any pixel-addressable graphics modes. It had only a single monochrome text mode (PC video mode 7), which could display 80 columns by 25 lines of high resolution text characters or symbols useful for drawing forms.

The standard IBM MDA card was equipped with four kilobytes (kB) of video memory. The MDA's high character resolution (sharpness) was a feature meant to facilitate business and wordprocessing use: Each character was rendered in a box of 9×14 pixels, of which 8×14 made out the character itself (the other pixels being used for space between character columns and lines). Some characters, such as the lowercase "m", were rendered eight pixels across.

The MDA featured the following character display attributes: invisible, underline, normal, bright (bold), reverse video, and blinking; some of these attributes could be combined, so that e.g., bright, underlined text could be produced.

The theoretical total screen display resolution of the MDA was 720×350 pixels. This number is arrived at through calculating character width (nine pixels) by columns of text (80) and character height (14 pixels) by rows of text (25). However, the MDA again could not address individual pixels; it could only work in text mode, limiting its choice of display patterns to 256 characters. Its character set is known as code page 437. The character patterns were stored in ROM on the card, and so could not be changed by software. The only way to simulate "graphical" screen content was through ASCII art.

IBM's original MDA included a parallel printer port (hence its original name of "Monochrome Display and Printer Adapter"), thus avoiding the need for a separate parallel interface on computers fitted with an MDA.

Specifications

Connector

Pin numbers (looking at socket):

Pin assignments^[1]

Pin	Function
1	Ground
2	Ground
3	Not Used
4	Not Used
5	Not Used
6	Intensity
7	Video
8	Horizontal Sync (+)
9	Vertical Sync (-)

Signal

Type	Digital, TTL
Resolution	720h × 350v
H-freq	18.432 kHz
V-freq	50 Hz
Colors	1
Color Intensity	2-4[2]

Competing adapters

There were two commonly available competing display adapters:

• For PC users requiring bitmapped graphics and/or color, IBM offered its Color Graphics Adapter (CGA, also CGA card), launched at the same time as their MDA. The lower resolution of its text mode characters (as compared to MDA) and absence of a printer port, which was included on the original MDA card, made CGA cards less attractive for business use.
• Introduced in 1982, the non-IBM Hercules Graphics Card (also HGC) offered both an MDA compatible high resolution text mode and a monochrome graphics mode. It could address individual pixels and display a black and white picture of 720×348 pixels. This resolution was better than even the highest monochrome resolution CGA cards could offer. Thus, even without a color capability of any kind, the Hercules adapter's offer of monochrome graphics without sacrificing MDA-equivalent text quality made it a more desirable choice for many.

See also

• Green-screen display
• List of display interfaces

References

1. ^ IBM Personal Computer Hardware Library: Technical Reference (Revised edition, 1983)
2. ^ There are four possible combinations of values for the 'Intensity' and 'Video' pins, but not all monitors will display them as four distinct intensity levels.

External links

• Monochrome Display Adapter Notes