Hardware Reference
In-Depth Information
Chapter 4. Motherboards and Buses
Motherboard Form Factors
Without a doubt, the most important component in a PC system is the main board or motherboard.
Virtually every internal component in a PC connects to the motherboard, and its features largely
determine what your computer is capable of, not to mention its overall performance. Although I prefer
the term
motherboard
, other terms such as
main board
,
system board
, and
planar
are
interchangeable. This chapter examines the various types of motherboards available and those
components typically contained on the motherboard and motherboard interface connectors.
Several common form factors are used for PC motherboards. The
form factor
refers to the physical
dimensions (size and shape) as well as certain connector, screw hole, and other positions that dictate
into which type of case the board will fit. Some are true standards (meaning that all boards with that
form factor are interchangeable), whereas others are not standardized enough to allow for
interchangeability. Unfortunately, these nonstandard form factors preclude any easy upgrade or
inexpensive replacement, which generally means that you should avoid them. The more commonly
known PC motherboard form factors include the following:
Obsolete Form Factors
• Baby-AT (PC and XT)
• Full-size AT
• LPX (semiproprietary)
• NLX
• WTX
• BTX, microBTX, picoBTX
Modern Form Factors
• ATX and variants; microATX, FlexATX, DTX/Mini-DTX, and ITX/Mini-ITX
PC motherboard form factors have evolved over the years from the Baby-AT form factor boards
based on the original IBM PC and XT to the current ATX form factor (and variants) used in most
desktop and tower systems. ATX has a growing number of variants, mostly in smaller sizes designed
to fit different market segments and applications. The short-lived BTX form factors relocated major
components to improve system cooling and incorporate a thermal module but failed to receive
widespread adoption.
Anything that does not fit into one of the industry-standard form factors should be considered
proprietary. Unless there are special circumstances, I do not recommend purchasing systems based on
proprietary board designs. They are difficult to upgrade and expensive to repair because components
such as the motherboard, case, and power supply are not interchangeable with other systems. I often
call proprietary form factor systems “disposable” PCs because that's what you must normally do with
them when they are too slow or need repair out of warranty.
Obsolete Form Factors
The following sections examine industry-standard motherboard form factors no longer in use but
