1U for racks? RE: [Beowulf] Newbie Question: Racks...

Lux, James P james.p.lux at jpl.nasa.gov
Fri Dec 12 10:35:33 PST 2008

        From: beowulf-bounces at beowulf.org [mailto:beowulf-bounces at beowulf.org] On Behalf Of John Hearns

        2008/12/11 arjuna <brahmaforces at gmail.com>

                What is 1u?

        Easy question! a "U" is short for a "rack unit". Rack mounted equipment always comes in multiples of a vertical height unit, which is 1.75 inches. I gather this is actually an old Russian unit of measurement (you can check on Wikipedia). So when you put equipment into standard 19inch wide racks, you ask "how many U high is that equipment). It means that you can mix and match different types of equipment in the same rack.

-- The Wikipedia entry just says it's coincidence: "Coincidentally, a rack unit is equal to a vershok, an obsolete Russian length unit."

I used to think that the rack hole spacing is almost certainly from Western Electric or something like this (back when they were called "relay racks").   The rack dimensions are an old RETMA standard ("Radio Electron Television Manufacturing Association) (RETMA changed to EIA in the late 50s) which I'm pretty sure derives from some older standard, which in turn probably derives from an early telegraphy standard, so it, could, like the stories about railroad gauge, be derived from the dimensions of Roman donkeys.

A usenet post from Larry Lippman in 1990 gives what sounds like a fairly authoritative description:
Ma Bell -> 23", 2" spacing (starting in 1917 or thereabouts)
Other older -> 19", 1.75" spacing (he thought RCA, perhaps, in the early 20s)

The web is a wonderful place to waste time.. Now I can amaze folks at work with a truly arcane piece of information.

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