Hardware Reference
In-Depth Information
has the same basic electrical connectors; the main difference is which type of power sup-
ply the case is physically designed to accept.
EPS/EPS12V
In1998,agroupofcompaniesincludingIntel,Hewlett-Packard,NEC,Dell,DataGeneral,
Micron,andCompaqcreatedtheServerSystemInfrastructure(SSI),anindustryinitiative
to promote industry-standard form factors covering common server hardware elements
such as chassis, power supplies, motherboards, and other components. The idea was to
be able to design network servers that could use industry-standard interchangeable parts.
You can find out more about SSI at
www.ssiforum.org
. Although this topic does not cov-
er network servers, in many ways a low-end server is a high-end PC, and many high-end
components that were once found only on servers have trickled down to standard PCs.
This trickle-down theory is especially true when it comes to power supplies.
In 1998, the SSI created the entry-level power supply (EPS) specification, which defines
an industry-standard power supply form factor for entry-level pedestal (standalone tower
chassis) servers. The initial EPS standard was based on ATX, but with several enhance-
ments. The first major enhancement was the use of a 24-pin main power connector,
which eventually trickled down to the ATX12V as well as other power supply form factor
specifications in 2003. EPS also originally called for the use of HCS terminals in the
Molex Mini-Fit Jr.-based power supply connectors, which became standard in ATX12V
in March 2005. In addition, the (now-obsolete) auxiliary 6-pin power connector, the 4-pin
+12Vpowerconnector,andavariationofthe6-pingraphicspowerconnectorallappeared
in the EPS specifications before ending up in ATX.
The EPS specification originally used a mechanical form factor identical to ATX, but the
EPS form factor was later extended to support higher power outputs by allowing the body
of the supply to be deeper if necessary. The ATX and the original EPS standards call
for a supply that is 86mm tall by 150mm wide by 140mm deep, the same dimensions as
the LPX or PS/2 form factors. EPS later added optional extended depths of 180mm and
230mm total. Most power supplies with true ratings of 500 watts or more are made in the
EPS12V form factor, because it isn't really possible to fit more power than that into the
standard ATX size. You may think these would require a custom EPS chassis, but in fact
many (if not most) full-sized ATX tower chassis can handle these greater depths without
interference,especiallywhenusingoneofthenewerbreedofshorter-lengthopticaldrives
(because one or more of the optical drives are often inline with the power supply).
With the improvements in EPS/EPS12V power supplies trickling down to ATX/ATX12V,
Ihave studied the SSIEPS specifications to see what potential improvements might come
to ATX. The main difference today between ATX and EPS with respect to connectors is
the use of an 8-pin dual +12V connector in EPS12V instead of a 4-pin +12V connector in
