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<TITLE>Re: [luau] Re: Networking Question</TITLE>
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<PRE>Warren,
I'm not too sure about the vernacular here, but my understanding is
that the way you wire ethernet cables for directly connecting two
computers is different from those cables used to connect to a hub or
router. The technician that built my cable for me called it a
"half-rolled CAT 5 cable." Basically he crossed two wires on the
terminating jack in the same manner that one would do to build a
null-modem cable. That way, the two ethernet cards believe that the
circuits are completed. The following is extracted from the Ethernet
HOWTO at http://www.linuxdoc.org/HOWTO/Ethernet-HOWTO-5.html
5.2 Twisted Pair
Twisted pair networks require active hubs, which start around $50, and
the raw cable cost can actually be higher than thinnet. You can pretty
much ignore claims that you can use your existing telephone wiring as
it is a rare installation where
that turns out to be the case.
On the other hand, all 100Mb/sec ethernet proposals use twisted pair,
and most new business installations use twisted pair. Also, Russ Nelson
adds that `New installations should use Category 5 wiring. Anything
else is a waste of your
installer's time, as 100Base-whatever is going to require Cat 5.'
If you are only connecting two machines, it is possible to avoid using
a hub, by swapping the Rx and Tx pairs (1-2 and 3-6).
If you hold the RJ-45 connector facing you (as if you were going to
plug it into your mouth) with the lock tab on the top, then the pins
are numbered 1 to 8 from left to right. The pin usage is as follows:
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
If you want to make a cable, the following should spell it out for you.
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
above table, you will see that 1+2 and
3+6 are the two sets of differential signal pairs. Not 1+3 and 2+6
!!!!!! At 10MHz, with short lengths, you *may* get away with such
errors, if it is only over a short length. Don't even think about it at
100MHz.
For a normal patch cord, with ends `A' and `B', you want straight
through 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.
Note that before 10BaseT was ratified as a standard, there existed
other network formats using RJ-45 connectors, and the same wiring
scheme as above. Examples are SynOptics's LattisNet, and AT&T's
StarLAN. In some cases, (as with
early 3C503 cards) you could set jumpers to get the card to talk to
hubs of different types, but in most cases cards designed for these
older types of networks will not work with standard 10BaseT
networks/hubs. (Note that if the cards also have
an AUI port, then there is no reason as to why you can't use that,
combined with an AUI to 10BaseT transceiver.)
Note the section for a normal patch cord.
Hope this helps,
Ben
>>>>>>>>>>>>>>>>>> Original Message <<<<<<<<<<<<<<<<<<
On 7/7/01, 9:32:29 PM, "Warren Togami" <warren@togami.com> wrote
regarding [luau] Re: Networking Question:
> What is a roll over cable anyway?
> ----- Original Message -----
> From: "Dusty" <dusty@sandust.com>
> To: "Linux & Unix Advocates & Users" <luau@list.luau.hi.net>
> Sent: Saturday, July 07, 2001 8:00 PM
> Subject: [luau] Re: Networking Question
> > 5. Is your cross-over cable configured correctly? A rollover is
different
> from a crossover. On standard ethernet we only use wires 1
> ---
> You are currently subscribed to luau as: beesond001@hawaii.rr.com
> To unsubscribe send a blank email to leave-luau-436Q@list.luau.hi.net</PRE>
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