Subject: Re: ethernet magic
To: None <port-vax@netbsd.org>
From: pcw <pcw@karpy.com>
List: port-vax
Date: 11/23/2001 10:40:51
>> Final possibility you aren't using a MAU cable, instead you've used a
>> generic 15 pin PC joystick type cable and that just won't work.
>
>It may, if it connects the relevant pins and is short enough.  I once
>did a MAU cable with two DB-15s and some 15-condutor ribbon cable.  I
>first used all the ribbon cable I had on hand (some 15-20 feet) and
>tested it; I saw occasional (a few percent) errors.  So I cut it down
>to about six inches and have never had any trouble with it.
>
>Of course, YMMV.  This was in a not particularly electromagnetically
>noisy environment....

Actually the main problem with flat cable is that it represenst a large
impedance mis-match --> ~130 ohms (depending on actual cable wiring)
versus 78 ohms for STP (which MAU drop cables are supposed to be) When you
get 10-20 feet of cable, you will get severe reflections...


a couple of feet will not be a problem (3 feet would be about 1/8 wave at
20 MHz - not a major discontinuity)


PCW

>
>/~\ The ASCII                           der Mouse
>\ / Ribbon Campaign
> X  Against HTML               mouse@rodents.montreal.qc.ca
>/ \ Email!           7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B