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IBM Monochrome Display Adapter

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Title: IBM Monochrome Display Adapter  
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IBM Monochrome Display Adapter

Monochrome Display Adapter
Release date 1981
Architecture Motorola 6845
Cards
Entry-level IBM MDA, Control Systems Artist 1, Hitachi HD6845SP, UMC UM6845
History
Successor Hercules Graphics Card, Enhanced Graphics Adapter
IBM 5151 monitor driven by a Monochrome Display Adapter
IBM Monochrome Display and Parallel Printer Adapter (MDA)
MDA Video card with Hitachi HD6845 (= Motorola MC6845).
MDA Video card.

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.[1]

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.

Because of the lack of pixel-addressable graphics, MDA owners could not play most graphics-based games. At least one game, IBM's One Hundred And One Monochrome Mazes ("Amazing fun for the whole family"), required MDA.[2] Code page 437 included the standard 127 ASCII characters but also another 127 characters like the aforementioned characters for drawing forms. Some of these shapes would later show up in Unicode as box-drawing characters. The characters were also used in early PC games such as early BBS door games, or games like Castle Adventure by Kevin Bales.

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.

Contents

  • Output capabilities 1
  • Specifications 2
    • Connector 2.1
    • Signal 2.2
  • Early boards 3
  • 4 Clone boards
  • Competing adapters 5
  • See also 6
  • References 7
  • External links 8

Output capabilities

Characters of code page 437

Text modes:

  • 80×25 characters with a 8×14 pixel font (effective resolution of 720×350)
  • 4 intensity level combinations (for text and background) for each character cell


Specifications

Connector

Pin numbers (looking at socket):

Pin assignments[3]
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[4]

Early boards

Early versions of the MDA board had hardware capable of outputting red, green and blue TTL signals on the normally unconnected video connector pins, theoretically allowing an 8-color display with a suitable monitor. The registers also allow the monochrome mode to be set on and off. Yet there was no software to actually control that feature.[5][6][7]

Clone boards[1][8]

Other boards offered MDA compatibility, although with differences on how attributes are displayed or the font used.

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.
  • Adapters exist(2014) to convert MDA into VGA for upgrading to LCD. Using a GBS-8219 with the video signal (to G) and the horizontal sync (to H/CS) and vertical sync (to V); set to a RGB(D), and Sync Separate(HV).

See also

References

  1. ^ a b "Monochrome Display Adapter Notes". Seasip.info. 2005-11-06. Retrieved 2014-06-19. 
  2. ^ Manes, Stephen (1984-01-24). "It's Not Easy Being Green". PC Magazine. p. 391. Retrieved 24 October 2013. 
  3. ^ IBM Personal Computer Hardware Library: Technical Reference (Revised edition, 1983)
  4. ^ 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.
  5. ^ Wilton, Richard (1987). Programmer's Guide To PC And PS/2 Video Systems. Microsoft Press. p. 51.  
  6. ^ "IBM 5150 - early version". Minuszerodegrees.net. Retrieved 2014-08-30. 
  7. ^ http://www.seasip.info/VintagePC/mda.html#regs
  8. ^ http://www.vgamuseum.info/component/content/article/59-vlasks-articles/index.php?option=com_customproperties&view=show&task=show&cp_made=&cp_bus=&cp_memsize=&cp_year=&cp_memory=&cp_family=&cp_cardtype=mda&cp_owner=&cp_directx=&cp_opengl=&cp_pipelines=&cp_manufacturer=&cp_process=&cp_text_search=

External links

  • Monochrome Display Adapter Notes
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