[1972] in linux-net channel archive
Re: Networking without a hub or a router/repeater
daemon@ATHENA.MIT.EDU (Paul Gortmaker)
Sun Feb 25 04:23:25 1996
From: Paul Gortmaker <gpg109@rsphy1.anu.edu.au>
To: mec@genesis.ezlink.com (Matt Clauson)
Date: Sun, 25 Feb 1996 14:59:03 +1100 (EST)
Cc: linux-net@vger.rutgers.edu
In-Reply-To: <199602240035.RAA00468@genesis.ezlink.com> from "Matt Clauson" at Feb 23, 96 05:43:28 pm
"Matt Clauson" at Feb 23, 96 05:43:28 pm
> >According to the Ethernet-HOWTO, one twists 1&2 and 3&6, not 1&2, 3&4,
> >5&6, and 7&8 as one might interpret this. See the Ethernet-HOWTO.
>
> This is almost correct. I'm running 10BaseT between my Linux box and my
> Mac with 1&3 and 2&6 twisted, Not 1/2 and 3/6. I think there may be a way
No. That is not correct. Differential signal pairs must be on the same
twisted pair to get the required minimal impedance/loss of a UTP cable.
If you look at the following table, you will see that 1&2 and 3&6 are
the two sets of differential signal pairs. Not 1&3 and 2&6 like you
use. At 10MHz, with short lengths, you can get away with such errors,
which is why yours works regardless.
Pin Number Assignment
---------- ----------
1 Output Data (+)
2 Output Data (-)
3 Input Data (+)
4 Reserved for Telephone use
5 Reserved for Telephone use
6 Input Data (-)
7 Reserved for Telephone use
8 Reserved for Telephone use
For those that have been e-mailing me asking how to do it, I will
spell it out as clearly as possible.
For a normal patch cord, with ends "A" and "B", you want straight thru
pin-to-pin mapping, with the input and output each using a pair of
twisted wires (for impedance issues). That means 1A goes to 1B,
2A goes to 2B, 3A goes to 3B and 6A goes to 6B. The wires joining
1A-1B and 2A-2B must be a twisted pair. Also the wires joining 3A-3B
and 6A-6B must be another twisted pair.
Now if you don't have a hub, and want to make a "null cable", what you
want to do is make the input of "A" be the output of "B" and the
output of "A" be the input of "B", without changing the polarity.
Tha means connecting 1A to 3B (out+ A to in+ B) and 2A to 6B
(out- A to in- B). These two wires must be a twisted pair. They carry
what card/plug "A" considers output, and what is seen as input
for card/plug "B". Then connect 3A to 1B (in+ A to out+ B) and also
connect 6A to 2B (in- A to out- B). These second two must also be
a twisted pair. They carry what card/plug "A" considers input, and
what card/plug "B" considers output.
So, if you consider a normal patch cord, chop one end off of it,
swap the places of the Rx and Tx twisted pairs into the new plug,
and crimp it down, you then have a "null" cable. Nothing complicated.
You just want to feed the Tx signal of one card into the Rx of the
second and vice versa.
Paul.
