[Tfug] Yet another poser

[johngalt1]

My junk box has a number of serial adapters also. But this is far from 
comprehensive. There are just too many possibilities. Lately, I have been 
rolling my own. I am fond of those DB9 or DB25 shells with RJ45 jacks molded 
in. If the device doesn't require handshaking, I don't bother. If transmit, 
receive and ground doesn't work, I put one of those passthroughs with the 
LEDs on each side and the device may give a clue what it wants. Cisco and 
others place the center conductors (4,5) as ground. Then, transmit and 
recieve share the next conductors out (3,6). If I need to change between DTE 
and DCE, I use an RJ45 cable that has a twist instead of straight through. 
Beyond that, it's not well thought out.

Regarding the Ethernet crossover issue, I pulled out a factory made cable 
bought at Graybar. It omits pairs other than the transmit and receive pairs. 
I put an "X" tag or sharpie mark on such cables.

----- Original Message ----- 
From: "Bexley Hall" <>
To: <tfug at tfug.org>
Sent: Friday, September 14, 2007 12:23 PM
Subject: [Tfug] Yet another poser


> OK, this is one of those philosophical questions
> that you either can't get *wrong* -- or, you can't
> get *right*!  :-/
>
> I am a big fan of "standardized" cables.  Having
> spent a few years designing communications gear,
> I quickly learned that keeping track of 4,928
> different versions of a "25 pin serial cable"
> was just *not* how I wanted to spend my career!
>
> And, looking through a box of 4,927 of those
> cables in the hope of finding the 4,928th
> variety to save myself the trouble of having to
> build one from scratch ranks right up there with
> waiting for a dentist on a holiday weekend to
> take care of an abcessed tooth!  :-(
>
> So, in as much as is possible, I have tried to
> rid myself of this problem by "picking" what I
> think is an appropriate "standard" cable and
> then building "widgets" to append to said
> cable to modify it's *overall* character.
>
> E.g., I have lots of M-F 25 pin "straight through"
> cables that I use for most serial and "parallel"
> cables.  My little box of widgets, for example,
> contains a device with two male connectors on it
> labeled "female gender swap" (i.e., it converts
> a female connector that it *mates* with into a
> *male* connector, effectively).  So, put one
> of these on a generic M-F cable and you end up
> with a M-M cable.  Likewise, put a "male gender
> swap" widget on the same generic cable and it
> becomes a F-F cable.
>
> Of course, the more *interesting* widgets are
> those that actually alter the effective wiring
> of the cable -- not just the gender of the
> connectors on its ends.  For example, one widget
> is a "null modem" which, of course, converts
> the generic cable into a "null modem cable".
> Another widget swaps the "port A" and "port B"
> connections on some older SPARC machines.  Still
> another converts the generic cable into one
> suitable for use as a "PLIP" cable.  :>
>
> <shrug>  YMMV.  This works *great* for me!
> (Note that I can get cables of arbitrary lengths
> just by cascading generic M-F cables onto the
> "widgetized" cable!  :> )  *Technically*, these
> "widgetized cables" aren't identical to "real"
> cables -- except at DC.  But, they are usually
> only "temporary use" so the differences never
> really amount to any *noticeable* performance
> hits, etc.
>
> Anyway, I wanted to do the same thing with
> network cables.  To that end, I have some little
> widgets with a pair of RJ45 connectors back-to-back
> (the key thing about widgets is they are *small*
> so you can store 50 different varieties in a
> shoebox -- try doing that with 50 different
> varieties of *cables*!).  They are wired
> "straight through".  As such, their only real
> use is as "cable extenders" -- i.e. plug two
> RJ45 cables into it and you end up with a cable
> that is as long as their combined lengths.
>
> It's a cakewalk to turn one of these "RJ45 widgets"
> into a "crossover" widget.  I.e. swap pins 1&3
> and 2&6, then glue the thing back together.
>
> The question becomes what to do with the other
> 4 conductors in the "widget".  I suppose to
> truly mimic a crossover cable, they should be
> present and run "straight through" -- is this,
> in fact, how "real" crossover cables are wired?
> Or, do vendors skimp and simply *omit* these
> extra conductors to save a few millipennies?
>
> Lastly, how are crossover cables typically
> identified?  I had assumed I would just slap a
> label on this (I label all of my widgets since
> you "can't tell the players without a program"!)
> saying "CROSSOVER"...?
>
> (Yeah, I think WAY too much about these little
> details... :>  )
>
> Thx!
>
>
> 
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