From the IBM PC in 1981 through the PC AT in 1984, IBM preferred to keep a minimum of features in the base unit. Instead, it allowed users to extend their systems with expansion cards that plugged into the internal slots. This meant that a 1981 PC, which shipped with five slots, left little room for expansion when it already contained a graphics card, a disk controller, a serial card, and a printer card--a common configuration at the time.
With the PS/2, IBM chose to integrate many of those commonly used I/O boards into the motherboard itself. Each model in the PS/2 line included a built-in serial port, parallel port, mouse port, video adapter, and floppy controller, which freed up internal slots for other uses.
Computers in the PS/2 series also had a few other built-in advancements, such as the 16550 UART, a chip that allowed faster serial communications (useful when using a modem), as well as 72-pin RAM SIMM (single in-line memory module) sockets. Both items became standard across the industry over time.
PS/2 Keyboard and Mouse Ports
The built-in mouse port I mentioned earlier is worth noting in more detail. Each machine in the PS/2 line included a redesigned keyboard port and a new mouse port, both of which used 6-pin mini-DIN connectors.
IBM intended the mouse, as a peripheral, to play a major part in the PS/2 system. The company promised a new graphical OS (which I'll talk about later) that would compete with the Macintosh in windowing functionality.
Even today, many new PCs ship with "PS/2 connectors" for mice and keyboards, although they have been steadily falling out of fashion in favor of USB ports.
New Floppy Drives
Every model in the PS/2 line contained a 3.5-inch microfloppy drive, a Sony-developed technology that, until then, had been featured most prominently in Apple Macintosh computers.
The low-end PS/2 Model 30 shipped with a drive that could read and write 720KB double-density disks. Other models introduced something completely new: a 1440KB high-density floppy drive that would become the PC floppy drive standard for the next 20 years.
IBM's use of the 3.5-inch floppy drive was new in the PC-compatible world. Up to that point, IBM itself had favored traditional 5.25-inch disk drives. This drastic format shift initially came as a great annoyance to PC users with large libraries of software on 5.25-inch disks.
Although IBM did offer an external 5.25-inch drive option for the PS/2 line, cloners quickly followed suit with their own 3.5-inch drives, and many commercial software applications began shipping with both 5.25-inch and 3.5-inch floppies in the box.