Hardware Reference
In-Depth Information
initial EPS standard was based on ATX, but with several enhancements. 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 +12V power connector, and a variation of the 6-pin graphics power connector all appeared 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, especially when using one of the newer breed of shorter-length optical drives (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, I have
studied the SSI EPS 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 ATX12V. The 8-pin dual +12V connector
is essentially the equivalent of two 4-pin connectors mated together and is used by entry-level servers
to power multiple processors. Because of the way the connectors are designed, an 8-pin +12V
connector can plug into a 4-pin +12V connector on a motherboard, with the unused pins simply
hanging unused, offset to one side or the other.
The only other major difference between EPS12V and ATX12V is that EPS power supplies can be up
to 180mm or 230mm deep, whereas ATX supplies are technically limited to 140mm depth according
to the specification. An example of an EPS12V type supply from PC Power & Cooling is shown in
