Xref: utzoo comp.misc:6857 misc.misc:7596
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ucsd!sdcc6!mplvax!cdl
From: cdl@mplvax.EDU (Carl Lowenstein)
Newsgroups: comp.misc,misc.misc
Subject: Re: RETMA definition wanted
Keywords: rails
Message-ID: <984@mplvax.EDU>
Date: 18 Aug 89 21:26:07 GMT
References: <447@qtc.UUCP>
Reply-To: cdl@mplvax.ucsd.edu.UUCP (Carl Lowenstein)
Organization: U.C. San Diego, Marine Physical Laboratory
Lines: 52

In article <447@qtc.UUCP> battan@qtc.UUCP (Jim Battan) writes:
>I hear that "RETMA rails" are the standard 19" rail spacing in all
>computer equipment racks.
>What does the "RETMA" acronym stand for?
Radio Electronic Television Manufacturers' Association

>Who defined the "standard"?
Nobody all at once, it just sort of happened.  Here comes some history
from memory, since I haven't seen the printed references in 25 years or
more.

First there was Western Electric (WE), who made equipment for telephone
systems.  They wanted to mount their relays in relay racks (naturally),
and chose to do so on panels that were multiples of 1.75 inches high.
To fasten the panels to the racks, a pattern of holes was devised that
repeated every 1.75 inches.  Pairs of holes 0.5 inches on center.  So
the over-all hole pattern was alternating 0.5 inch and 1.25 inch
spaces.  Each panel fit with its edges between a pair of 0.5-inch
spaced holes.

Next there was the radio broadcasting industry, who also wanted to
mount things neatly in racks.  Perhaps not electronic things, since the
word Electronics did not become widely used until the mid to late
1930's.  Nevertheless, the Radio Manufacturers' Association (RMA) chose
to use panels which were a multiple of 1.75 inches, but they decided on
a hole pattern for the rack which was just a uniform 1.75 inches
everywhere.  Each panel fit with its edges between a pair of 1.75-inch
spaced holes.

Radio amateurs, who scrounged equipment from everywhere, soon noticed
that WE panels would only sometimes fit RMA racks, and vice versa.
Eventually, there was a great compromise when somebody noticed that a
rack could be drilled with the inclusive OR of the two hole patterns.
So it is unto this day that the 1.75 inch repetition of the hole
pattern consists of 0.5 inch, 0.625 inch, 0.625 inch.  And the edge of
the panel still fits between the 0.5-inch spaced holes, which happens
to be also the mid-point of a pair of 1.75-inch spaced holes.

Along the way, the RMA diversified itself, as there came to be
Electronics and Television, so there was the Radio Electronics
Television Manufacturers' Association (RETMA).  Eventually they gave up
on adding new nouns to their title, and ended up as the Electronic
Industries' Association (EIA).

And this, my dearly beloved, explains why some equipment still doesn't
fit in its rack. It is off by 0.25 inch one way or the other, because
somebody doesn't know the difference between the 0.5-inch hole pairs
and the 0.625-inch hole pairs.
-- 
	carl lowenstein		marine physical lab	u.c. san diego
	{decvax|ucbvax}	!ucsd!mplvax!cdl
	cdl@mpl.ucsd.edu