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From: tom@wcc.oz (Tom Evans)
Newsgroups: comp.dcom.lans
Subject: Re: Req For Info - Ethernet Electrical Rules/Specs
Message-ID: <369@wcc.oz>
Date: 10 Oct 89 07:40:42 GMT
References: <188.2526de30@acci.com> <580@trwind.UUCP>
Organization: Webster Computer, Melbourne, Australia
Lines: 24

In article <580@trwind.UUCP>, johng@trwind.UUCP (John Greene) writes:
> In article <188.2526de30@acci.com> ta2@acci.com writes:
> >
> >1) Why can't you use 75ohm cable for thin ethernet?
> 
> You would be creating an impedance mis-match with the Thin Ethernet
> transceiver. This will cause relections on the cable (transmission line)
> and distort the signal.

There's a far worse problem. The Ethernet transmitters drive a
constant CURRENT into the cable of 90 +-4 mA. This develops a VOLTAGE
across the cable that the receivers receive. If they don't see an
average voltage of half of the peak 90e-3 * (50/2), they scream
"COLLISION!!". The actual allowed range is -1.492V to -1.629V.

A 75 ohm cable will develop 0.5 * (90e-3 * (75/2) ) = 1.69V average
voltage, and will trigger the collision detect. Ergo elk.
			    ---------
Tom Evans  tom@wcc.oz		|
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