From: [email protected] [mailto:[email protected]]
On Behalf Of John Hearns
2008/12/11 arjuna <[email protected]>
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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