When you hear about the PS/2, you ask yourself one logical question: was there ever a PS/1? And yes, there was...
By 1987, IBM had lost a large percentage of its market share to competitors whose products were cheaper yet fully compatible with IBM's PC lineup. These competitors were successful in small businesses and homes where the IBM name did not matter, but price and software compatibility did.
The PCjr, released in 1984, was IBM's last attempt to bring their computers in small businesses and homes. The PCjr offered additional features which made it more complete for the traditional small business or home user which may not have technical know-how. Also, for people buying a PCjr for home use, the machine contained cartridge slots, RF modulated video/audio output to enable a TV to be used as a display, and even an early chicklet-style wireless keyboard known as the "Freeboard."
However, in 1985, IBM was once again left without a consumer-grade machine after the PCjr failed. The PS/2 was designed as a computer targeted at medium to large businesses, with plenty of expansion, features, and power. However, the PS/2 was priced within a range which made it difficult for home and small business users to purchase, not including the confusion of over 20 different models.
In 1990, IBM came up with a simple way to cater to the needs of their home and small-business users, with the Personal System/1 (PS/1). The PS/1 was originally an AIO (all-in-one) package, with the color monitor and system unit packaged into one system (and connected, externally, through thick cables. The power supply was located in the monitor section, meaning that both had to be connected for the system to work.) This was similar to Apple's Compact Macs of the time. (Although the Macs were more self-contained and the monitor and system unit was not in split sections.) Later models were split units, and very similar to the later Aptiva models. (The Aptiva may have, in fact, been modeled closely after the later PS/1 models.)
The PS/1 ran IBM's PC-DOS, their competitor to MS-DOS. The PS/1 also featured a graphical menu (a.k.a. "4-quad"), making it easier to use. You could run Microsoft Windows or OS/2 on the PS/1, but it was not recommended as the system would become slow and unstable, or not work at all. The early PS/1 models had the 4-quad and PC-DOS stored in ROM (see below), but later models had it stored on the hard drive.
Under the Hood...
The PS/1, much like the PS/2, consisted of various models which all contained different specifications and internal hardware configurations. Earlier models featured the Intel 80286 clocked in at 10MHz, whereas the latest models, before the PS/1 was discontinued in 1994, used the Intel 486 DX2 at 66 MHz.
DOS in ROM
Most early PS/1 models featured "DOS in ROM", where the disk operating system was located on a ROM chip, rather than a hard drive or a set of floppy diskettes. This approach was somewhat common of home computers of the era, with Tandy also using the "DOS in ROM" approach on some of their Tandy 1000 models.
Other computers of the era also placed the power supply in strange locations so that you had to have the "complete set." The Coleco Adam had its power supply located in the printer, and the Amstrad PC1512 had the power supply located in its monitor. Many PS/1 models followed the Amstrad approach by locating the power supply in the monitor, and using a proprieotary connector to connect the power supply to the CPU unit.