[rescue] D-shell nomenclature (was Netra T1 AC200 Questions)

der Mouse mouse at Rodents-Montreal.ORG
Sun Feb 14 21:45:29 CST 2010

> how to build an RJ45 to DB9 (I'm assuming it's DB9, although I've
> also heard DE9 for a standard PC serial connector)

It's properly DE.

The letter indicates the size of the shell; a DB connector has the same
size as the 25-pin connector that is standard for serial ports (yes,
RS-232 specifies the connector as part of the standard) and also used
for various other things, such as peecee parallel ports and older
Macintosh SCSI.  DB is usually used with 25 pins (or at least pin
positions), but sometimes other things, such as the way Sun and SGI and
probably others put three co-ax connectors and 10 pins in a DB shell
for video (and usually call it 13W3).

However, a lot of people make the same mistake I did for years and use
DB for all of them; I've even seen vendor catalogs exhibit this
misunderstanding.  However, this loses information; for example, under
that nomenclature, a "DB-15" could mean either the one properly called
DA-15 (used for peecee joystick/MIDI ports and, with a slide latch, for
AUI Ethernet) or the one properly called DE-15 (the higher-density one
used commonly for video on peecees and very little else).

The order of the letters is a little odd.  DA is the 15-pin one, DB the
25-pin one, DC is almost never used and I'm not even sure how many pins
it has room for at the usual spacing (37 or 39, I think), and DD is a
large one that usually holds 50 pins in three rows, used by old Suns
for SCSI and IPI, and probably other things by various people.  DE is
the small one, most often with 9 pins as used by peecee serial ports,
but also used with 15 pins in a higher-density three-row arrangement
for video.  I conjecture that someone specced out A, B, C, D in
increasing size, then realized too late that something smaller than A
was needed, couldn't retcon the existing names, and wasn't geeky (or
perhaps daring) enough to call the new one D at .

Other sizes of D-shell connectors exist, but are relatively rare.  The
only example I can think of offhand is a 19-pin variant that fits in
between DA and DB in the DE-DA-DB-DC series, used by NeXT for carrying
video and power from CPU to the human interface and carrying keyboard
and mouse input the other way (and possibly other things - eg, I think
sound may have been included on it; it's been a while).

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