For those who did not use IBM-compatible PCs in the 1980s and 1990s, it may be surprising to hear that there were many, many possible standards of monitors, video cards, and video modes. And many of these were incompatible in both documented and undocumented ways. To start with, we had the IBM-defined original video standards:
- MDA
- This was defined by the original IBM Monochrome Display adapter, commonly found in the IBM 5150 PC, 5160 XT, and 5170 AT. It was capable of monochrome text only, in 80 columns by 24 lines.
- https://www.seasip.info/VintagePC/mda.html
- CGA
- The first CGA card was the IBM Color Graphics Adapter, giving the standard its name. CGA was commonly found in the same IBM PCs as MDA, but was also common in early clone PCs, XTs and ATs, as it was significantly cheaper to implement than EGA. CGA was capable of 40 and 80 column text, and multiple graphics modes from 160x100 to 640x200 with monochrome, 2, or 4 colors possible, depending on the mode. The original IBM card, and some clones, supported composite output with 16 colors, at low resolution.
- https://www.seasip.info/VintagePC/cga.html
- PC/jr
- The introduction of the IBM PC/jr in 1983 added support for two new video modes to the CGA standard, 160x200 and 320x200 with 16 colors and 640x200 with 4 colors. This standard would become much more widely used with the introduction of the Tandy 1000 series of personal computers, by Radio Shack.
- https://en.wikipedia.org/wiki/IBM_PCjr#Video
- EGA
- This standard was created by IBM's Enhanced Graphics Adapter. The EGA card was backwards-compatible to both CGA and MDA, and added support for up to 64 colors (with additional video RAM) and resolutions of up to 640x350. There were also 2 RCA-style connectors on the backplate, but they were not designed for CGA-style composite output.
- https://en.wikipedia.org/wiki/Enhanced_Graphics_Adapter
- PGA
- The IBM Professional Graphics Adapter, which supported 640x480 with 256 colors out of a palette of 4096. This is quite a rare card, mostly used for CAD drafting and other high end graphics applications on the IBM PC, XT, and AT. It requires a dedicated monitor, and is not compatible with VGA. It does feature CGA backwards compatibility.
- https://www.seasip.info/VintagePC/pgc.html
- VGA
- This is the IBM Video Graphics Array, which defined the standard for 16 color graphics at a resolution of 640x480 pixels, and 256 colors at 320x200. It produced analog output, rather than the digital output of the MDA, CGA and EGA cards, and used a DE-15 connector, rather than the DE-9 connector used for MDA, CGA and EGA. VGA was backwards-compatible with MDA, CGA and EGA modes, while not being able to be connected to RGBI digital monitors.
- https://en.wikipedia.org/wiki/Video_Graphics_Array
- 8514/A
- IBM introduced the 8514/A adapter in 1987, and it extended the VGA standard with basic graphics acceleration, and a 1024x768 (interlaced) graphics mode. It was available only for the PS/2 line of systems, until compatible clone versions of the card became available.
- https://en.wikipedia.org/wiki/IBM_8514
- XGA
- IBM introduced the XGA display standard of 1024x768 (non-interlaced) pixels at 256 colors, in 1990, again extending the VGA standard. Additionally, XGA (which was again only available for PS/2 systems, supported 64k colors at 640x480, which was a first for IBM original video cards on the PC. Interestingly, XGA, like the 8514, lacked support for 800x600, which was quite common in VGA clone video cards, at the time. There was a minor update to XGA, called XGA-2, which added support for 256 grey scale colors, and finally the addition of 800x600, with up to 64k colors.
- https://ardent-tool.com/video/XGA1.html
And then, we had a variety of third-party created standards:
- Hercules monochrome
- The Hercules Graphics Card was introduced in 1982, having been designed so that its creator could edit his masters thesis in the Thai language. It provided very high resolution (for the time) monochrome graphics at 720x348 and backwards compatible text modes with the IBM MDA card. It was extremely popular, and clones of this card were found in many PCs up until the release of the 486, when most computers were supplied with VGA.
- https://en.wikipedia.org/wiki/Hercules_Graphics_Card
- https://www.seasip.info/VintagePC/hercplus.html
- Plantronics ColorPlus
- This card was a CGA-compatible video card that added support for 16 colors at 320x200 and 4 colors at 640x200. It was not widely supported, but a surprising number of CGA clone cards supported this standard.
- https://www.seasip.info/VintagePC/plantronics.html
- Tandy Graphics
- Tandy's 1000 line of personal computers were very similar to IBM's failed PC/jr line, in both graphics and sound. They popularized the 320x200 16 color graphics mode, which was very widely supported in video games for DOS.
- https://en.wikipedia.org/wiki/Tandy_Graphics_Adapter
- SVGA
- SVGA was created by the Video Electronics Standards Association (VESA), founded by NEC, in 1988. It popularized the 800x600 video mode, at 16 and 256 colors, and then went on to create official, rather than de-facto, standards for PC-compatible video output formats.
- https://en.wikipedia.org/wiki/Super_VGA
There were also a number of other video output standards, most either clones that enhanced CGA, EGA, or VGA.
Is it a wonder that nobody understands IBM PC graphics? What a huge list of somewhat mutually incompatible standards there were!
