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Path: utzoo!mnetor!seismo!gatech!amdcad!phil
From: phil@amdcad.AMD.COM (Phil Ngai)
Newsgroups: comp.dcom.lans
Subject: Re: Thin Ethernet drop cables
Message-ID: <16350@amdcad.AMD.COM>
Date: Thu, 23-Apr-87 23:13:08 EST
Article-I.D.: amdcad.16350
Posted: Thu Apr 23 23:13:08 1987
Date-Received: Sat, 25-Apr-87 13:09:21 EST
References: <207@nih-csl.UUCP> <43715@beno.seismo.CSS.GOV>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Distribution: world
Organization: Advanced Micro Devices, Inc., Sunnyvale, Ca.
Lines: 36
Keywords: Ethernet thin-net cheapnet cheapernet

In article <43715@beno.seismo.CSS.GOV> foster@beno.CSS.GOV.UUCP (Glen Foster) writes:
>Can anyone tell me if the wires in a xcvr cable are "special" in any
>way?  If I were to make up a cable from high quality "RS-232C" "type"
>cable (AWG 22 I think, maybe 24's) would I have a reasonable chance of
>success?  The xcvr cable conductors appear to be AWG 20 to my untrained
>eye, perhaps if I doubled the 22/24's?

The considerations in the xcvr cable are:

1) dc resistance. you have to power the transceiver

2) ac signal attenuation. can't let the signal get too small.
note that you are driving lines terminated with 78 ohms

3) impedance. the characteristic impedance of the pairs (they are
twisted, aren't they?) shouldn't be too much different than 78 ohms or
reflections may be a problem

4) shielding. you don't want too much cross talk between the transmit
pair and the receive pair.

I wouldn't worry about esoteric stuff like velocity of propagation.

The requirements are all documented in the Ethernet book. The big
questions are 1) do you have shielded twisted pairs 2) how long is the
run? Assuming you do have shielded twisted pairs, I would look up the
spec and then see how the gauge of the existing wire compares with the
spec. If it isn't too outrageously different, I'd make up a cable and
try it. If it is too long but not more than twice as long, I'd try
doubling up the wires. Since you have 4 signals and only 12
conductors, I'd probably leave Collision as a single. 

Best thing is to try one and see if it works.

-- 
Phil Ngai, {ucbvax,decwrl,allegra}!amdcad!phil or amdcad!phil@decwrl.dec.com