Editing IBM Personal Computer (section)

=== Expansion ===
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[[File:Personal_computer_-_Museo_scienza_tecnologia_Milano_14617_02.jpg|alt=|thumb|The back of a PC, showing the five expansion slots]]
The expansion capability of the IBM PC was very significant to its success in the market. Some publications highlighted IBM's uncharacteristic decision to publish complete, thorough specifications of the system bus and memory map immediately on release, with the intention of fostering a market of compatible third-party hardware and software.<ref>{{Cite book|url=https://archive.org/stream/byte-magazine-1982-01/1982_01_BYTE_07-01_The_IBM_Personal_Computer#page/n57/mode/2up|title=Byte Magazine Volume 07 Number 01 - The IBM Personal Computer|date=January 1982}}</ref>

The motherboard includes five 62-pin card edge connectors which are connected to the CPU's I/O lines. IBM referred to these as "I/O slots", but after the expansion of the PC clone industry they became retroactively known as the [[Industry Standard Architecture|ISA bus]]. At the back of the machine is a metal panel, integrated into the steel chassis of the system unit, with a series of vertical slots lined up with each card slot.

Most expansion cards have a matching metal bracket which slots into one of these openings, serving two purposes. First, a screw inserted through a tab on the bracket into the chassis fastens the card securely in place, preventing the card from wiggling out of place. Second, any ports the card provides for external attachment are bolted to the bracket, keeping them secured in place as well.

The PC expansion slots can accept an enormous variety of expansion hardware, adding capabilities such as:

* Graphics
* Sound
* [[Computer mouse|Mouse]] support
* Expanded memory
* [[Joystick]] port
* Additional serial or parallel ports
* Networking
* Connection to proprietary industrial or scientific equipment

The market reacted as IBM had intended, and within a year or two of the PC's release the available options for expansion hardware were immense.

==== 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.