Editing IBM Personal Computer (section)

==== 5161 Expansion Unit ====
The expandability of the PC was important, but had significant limitations.

One major limitation was the inability to install a hard drive, as described above. Another was that there were only five expansion slots, which tended to get filled up by essential hardware - a PC with a graphics card, memory expansion, parallel card and serial card was left with only one open slot, for instance.

IBM rectified these problems in the later XT, which included more slots and support for an internal hard drive, but at the same time released the 5161 Expansion Unit, which could be used with either the XT or the original PC. The 5161 connected to the PC system unit using a cable and a card plugged into an expansion slot, and provided a second system chassis with more expansion slots and a hard drive.