The IBM PS/2: 25 years of PC history

Here's a fond look back at the Personal System/2 series of PCs, which embarrassed IBM in the 1980s but shaped the modern PC you know today.

By Benj Edwards, PC World |  Hardware, BIOS, compaq

Next Page: More PS/2 Features Explained

VGA and MCGA

In many ways, the PS/2 line is most notable, historically, for its introduction of the Video Graphics Array standard.

Among its many modes, VGA could display 640-by-480-pixel resolution with 16 colors on screen, and a resolution of 320 by 200 pixels with 256 colors, which was a significant improvement for PC-compatible systems at the time. It was also fully backward-compatible with the earlier Enhanced Graphics Adapter and Color Graphics Adapter standards from IBM.

In addition, the PS/2 line introduced what we now colloquially call a "VGA connector"--a 20-pin D-type socket that also became an industry standard.

The low-end Model 30 shipped with an integrated MCGA graphics adapter that could display a resolution of 320 by 200 pixels with 256 colors as well, but could display only 640 by 480 pixels in monochrome and was not backward-compatible with EGA. MCGA met its end after IBM included it in only a few low-end versions of the PS/2; cloners never favored it.

Micro Channel Architecture

The crowning glory of the PS/2 line's hardware improvements was supposed to be its new expansion bus, dubbed Micro Channel Architecture. Every initial PS/2 model except the low-end Model 30 shipped with internal MCA slots for use with expansion cards.

The Model 30 included three ISA expansion slots--the type used in the original IBM PC and extended for the PC AT line. Not surprisingly, the rest of the PC-compatible industry utilized the ISA expansion bus as well, so any PC-compatible machine could use almost all the cards created for other PC compatibles.

With the PS/2, IBM saw the opportunity to create an entirely new and improved expansion bus whose design it would strictly control and license, thus limiting the industry's ability to clone the PS/2 machines without paying a toll to IBM.

ISA had become slow and limiting by mid-1980s standards. MCA improved on it by increasing the data width from 16 bits to either 16 bits or 32 bits (which allowed more data to transmit over the bus at a time) and by improving the bus speed from 8MHz to 10MHz.

MCA also introduced a limited form of plug-and-play functionality, wherein each expansion card carried with it a unique 16-bit ID number that a PS/2 machine could identify to help it automatically configure the card.


Originally published on PC World |  Click here to read the original story.
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