[ILUG] Linux on a SparcStation

[Brian Foster]

  | Date: Wed, 8 May 2002 18:45:19 +0100
  | From: "Kenn Humborg" <kenn at bluetree.ie>
  | 
  |  > On Wed, May 08, 2002 at 06:36:47PM +0100, Mel wrote:
  |  >  >[...] how can I tell if it's a null cable or not?
  |  > 
  |  > Either you get lucky and have it written on the cable, or you get a
  |  > multi-meter and see if pins 3 & 4 are swapped (from memory - and it's
  |  > flaky memory.)
  | 
  | It's pins 2 and 3.
  | 
  | However, the standards for 9-way and 25-way connectors have the
  | opposite meanings, so a straight through cable has pin 2 on the
  | 9-way connected to pin 3 on the 25-way, while a null modem
  | (or crossover cable) has pin 2 on both ends connected together,
  | and pin 3 on both ends connected.
  | 
  | Real simple, this RS232 stuff :-)

 whilst DB-25 connectors are indeed standardised (at least
 de facto if not de jour), historically there are a number
 of DB-9 connector conventions floating around --- albeit
 (from memory) I believe what Kenn's outlined is the most
 common (as in IBM's PC convention).

 the approach to tackling this mess I've always advocated
 and implemented is to settle on a "unit standard cable",
 which is then built-up into whatever you need by using
 appropriate adaptors.  e.g., your "unit standard cable"
 might be DB-25 male on one end, DB-25 female on the other,
 connecting pins 2, 3, and 7 (at least, the flow- and modem-
 control circuits are also recommended) straight-through,
 shielded, with the shielding earthed only at the male end.
 ( for cables, it's important not to earth the shield at both
  ends; for adaptors, it's important to earth at both ends,
  as you want a continuous shield from the earthed end to
  the other end of a fully-built "cable" --- assuming you
  bother with shielding, that is. )

 to the extent possible, your adaptors change as few of
 the "unit standard cable"s properties as possible.
 e.g., your "unit standard null modem" would always be
 DB-25 male<->female, but swaps pins 2 and 3 rather than
 straight-through.  DB-25/DB-9 convertors are a mess, as
 you'll wind up needing all four sex combinations in a
 fully-equipped cabling kit; but none are null.

 then, in a situation like that here, where a null modem
 is needed ('cuz two DTEs are being connected) with one
 DB-9 end (of unspecified sex), a DB-25<->DB-9 converter
 of appropriate sexes plus a null modem would be tacked
 on (using the conventions above, could but needn't be
 at the same end), and you've the cable you need .....

 when you don't know what cable you need, a breakout box
 (nominally male DB25<->DB25 female) is invaluable.  the
 better breakout boxes tend to have both male and female
 DB-25 connectors on both sides, which can help.   for the
 more adventurous, a cable-making kit is useful for building
 any adaptors you need (as short cables), but I must admit
 these days I tend to just buy the convertors/adaptors I
 need (and to pick a pre-made cable as my "unit standard",
 making sure there are equivalent models are available from
 other suppliers).

 it's helpful to label the various adaptors, albeit the only
 cables you need label are those which aren't your chosen
 "unit standard".  (you always seem to have a few!)  note
 that because the adaptors generally change only one property
 at a time, you can often guess what the adaptor does simply
 by looking at it.  e.g., a male<->male DB-25 is simply that
 (not null; a null would be female<->male); a DB-25<->DB-9
 is simply that (not null; null modems are DB-25 at both
 ends); and so on.

 it may be helpful to have various lengths of your chosen
 "unit standard" cable, but note that if carefully designed,
 multiple such cables can be plugged directly together to
 get a longer "unit standard cable" (except the shielding
 is broken.....  ;-(  ).

cheers!
	-blf-

p.s. I've no idea if it's still in print, and I've long
     still lost my copy, but there used to be a good,
     and short, book(let) on RS-232 called something
     like ``All You Need to Know about RS-232'' ....

--
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