Hardware Reference
In-Depth Information
models.
If you can't tell already, I am a fan of the industry-standard form factors! Having standards and then
following them allows us to upgrade and repair our systems by easily replacing physically (and
electrically) interchangeable components. Having interchangeable parts means that we have a better
range of choices for replacement items, and the competition makes for better pricing, too.
In the PC market, IBM originally defined the form factor standards, and everybody else copied them;
this included power supplies. All the popular PC power supply form factors up through 1995 were
based on one of three IBM models, including the PC/XT, AT, and PS/2 Model 30. The interesting
thing is that these three original IBM power supply form factors had the same motherboard connectors
and pinouts; where they differed was mainly in shape, maximum power output, the number of
peripheral power connectors, and switch mounting. PC systems using knock-offs of one of those three
designs were popular up through 1996 and beyond; in fact, even the current industry standard
ATX12V models are based on the PS/2 Model 30 physical form factor, but with different connectors.
Intel defined a new power supply form factor in 1995 with the introduction of the ATX form factor.
ATX became popular in 1996 and started a shift away from the previous IBM-based standards. ATX
and the standards that have followed since use different connectors with additional voltages and
signals that allow systems with greater power consumption and additional features that would
otherwise not be possible with the AT-style supplies.
Note
Although two power supplies can share the same basic design and form factor, they can differ
greatly in quality and efficiency. Later in this chapter, you'll learn about some of the features
and specifications to look for when evaluating PC power supplies.
More than 10 different power supply form factors have existed that can be called industry standards.
Many of these are or were based on designs IBM created in the 1980s, whereas the rest are based on
Intel designs from the 1990s to the present. The industry-standard form factors can be broken down
into two main categories: those that are currently in use in modern systems and those that are largely
obsolete.
Note that although the names of some of the power supply form factors seem to be the same as those
of motherboard form factors, the power supply form factor relates more to the system chassis (case)
than to the motherboard. That is because all the form factors use one of only two main types of
connector designs: either AT or ATX, with subtle variations on each. So, although a particular power
supply form factor might be
typically
associated with a particular motherboard form factor, many
other power supplies would plug in as well.
For example, all modern ATX form factor motherboards with PCI Express slots have two main
power connectors, including a 24-pin ATX main connector along with a 4-pin +12V connector. All
the modern power supply form factors include these same connectors and therefore are capable of
plugging into the same motherboards. In other words, no matter what the form factor of the
motherboard (ATX, BTX, or any of the smaller variants of either), virtually any of the modern
industry-standard power supplies will plug in.
Plugging the power supply connectors into the motherboard is one thing, but for the power supply to
work in the system, it must physically fit inside the chassis or case—and that is what the different
