5150: Setting up MDA/CGA (without having an IBM monitor available)


Old monitors do wear out and they are difficult to build (a CRT, with lots of glass, and intricate components). It is much more practical to adapt old video signals into more modern display devices.

The original MDA and/or CGA video cards of the IBM PC use a 9-pin connector. More modern VGA monitors use a 15-pin connector. Be aware that “VGA” can refer to both a set of 256-color video modes, and “VGA” can also refer to the type of cable connection. So, the meaning will depend on the context of how it is used.

VGA as a connection style is largely phased out, now with DVI, HDMI, and DisplayPort connectors (which support even higher resolutions of video). Adapting the legacy MDA/CGA signals to a VGA monitor is not an easy process, so don’t expect an inexpensive $10 dongle-type adapter. Instead you will need to find a powered adapter, such as one of the following:

I do prefer the MonoTech since the buttons and dip-switches are more apparent and labeled on its case. But the texElec version I received may be a little easier to mount onto an electronics project board.

These are “made-to-order” devices that may be built in limited-run batches. They aren’t massed produced in a factory. The PCB spec is generally available online (github). Don’t be disappointed if they take awhile to order, and appreciate that they may have been hand built by someone just for you.

These devices won’t come with a VGA cable, so you will need that as well, and a CRT or LCD that has a VGA input. They might come with an MDA cable, double check during the order. If you want to adapt to an even more modern HDMI or DisplayPort, then an additional adapter will be needed. Some televisions may have a VGA input.

Variations of CGA Cards

BELOW: IBM PC 5150 CGA model 1804472 (with original black plate for IBM 5150 5-slot systems, this is an “old” model CGA)

BELOW: IBM PC CGA model 1501982 Color Graphics Adapter (newer model from 1983/1984)

BELOW: CGA-compatible clone from 1984, using the same MC6845 chip as earlier CGAs. This card was made in USA by STB Systems, which is a graphics company that merged with 3Dfx in 1999 (the makers of the legendary Voodoo graphics cards at around that time). The composite output on this card is monochrome only, no color.

An MDA video card and CGA video card independently output to two separate monitors. Often what I did was to have a TSR (Terminate Stay Resident) background program to output the current time (as well have an alarm clock feature) to the MDA screen, while doing the more typical programs and applications on the color monitor. But that was in a newer AT or Tandy 1000 setup with 8 slots available, it’s not practical in the IBM 5150 with only 5 slots.

Composite Mode CGA

The original CGA offers three video outputs:

(1) 9-pin “CGA” RGB connector (same style as used for MDA),
(2) Component (RCA), and
(3) internally it has pins to support an RF modulator (these pins were later used in the portable IBM PC, but their lineage is found also in the priori IBM 5100 that supported a RF output).

The composite output video of CGA is actually capable of many colors, but it was a feature that many early developers never took advantage of. There are a number of reasons for this, which take quite a while to explain in the context of the state of the industry in the early 1980s. The three main reasons were: (1) From the software developer side, the modes weren’t as well known or understood, since they weren’t (as well) documented for CGA. (2) Video card manufacturers (non-IBM clones) didn’t implement these modes consistently (if at all), (3) The capability wasn’t very well understood by consumers. This last reason is one that takes some explanation.

The IBM PC (of the early 1980s) wasn’t a “game system” like from Atari, ColecoVision, IntelliVision, or early Apple systems (which were expected to be connected to a television set, and had poor “text” font resolution as a consequence). Consumers wanted crisp text for Word Processor, so most opted for the monochrome MDA monitor, and the computer was physically in a home office (a room away from the living room “main TV”). The very idea of “gaming on a PC” just wasn’t yet fashionable, and from the consumer perspective (who had invested in the expensive high resolution monitor) there was no use or thought towards using the Component output of the PC (there was no nearby TV to connect it to anyway, there was no software that really took advantage of it, as televisions couldn’t show nearly as crisp text).

Many software titles did use 16-color Composite CGA modes, as that was successfully understood and standardized. But it was the above combination of reasons on why the more “advanced” modes were not used: few developers knew about it (and wanted to sell software to as many systems as possible, not focus on a specific niche capability), CGA clone manufacturers didn’t support it, and few consumers would have utilized it (for the practical reasons of not being inclined to connect their PC to a television, because the text-mode resolution was so poor).

For a list of games that supported Composite CGA output (as early as 1982), refer to the following:

Video Modes Supported : CGA Composite (16 colors) (mobygames.com)

See also PC Paint (1984) and PC Paintbrush (1985).

By the late 1980s, EGA (and then soon after, VGA), became more official standards and a large quantity of consumers had upgraded to that hardware. The very idea of using Component output on a PC CGA fell to the wayside, until it was resurrected in big a way decades later around 2015 with the 8088 Domination and 8088 MPH demos.

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