Hardware Reference
In-Depth Information
enough power to properly operate the new board, or it has an improperly wired or timed
Power_Good signal (which is more likely). In these situations, replacing the supply with
a higher-quality unit, in addition to the new motherboard, is the proper solution.
Power Supply Form Factors
The shape and general physical layout of a component is called the
form factor
. Items
that share a form factor are generally interchangeable, at least as far as their sizes and fits
are concerned. When designing a PC, the engineers can choose to use one of the popular
standard PSU form factors, or they can elect to build their own custom design. Choos-
ing the former means that a virtually inexhaustible supply of inexpensive replacement
parts will be available in a variety of quality and power output levels. Going the custom
routemeansadditionaltimeandexpensefordevelopment.Inaddition,thepowersupplyis
uniquetothesystemandgenerallyavailableforreplacementonlyfromtheoriginalmanu-
facturer.Thisprecludesanyupgradesaswell,suchasinstallinghigher-outputreplacement
models.
Ifyoucan'ttellalready,Iamafanoftheindustry-standardformfactors!Havingstandards
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
factorshadthesamemotherboardconnectorsandpinouts;wheretheydifferedwasmainly
inshape, maximum poweroutput, the number ofperipheral powerconnectors, andswitch
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
formfactor.ATXbecamepopularin1996andstartedashiftawayfromthepreviousIBM-
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.
