Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mcnc!decwrl!uunet!isavax.isa.com!cliffb
From: cliffb@isavax.isa.com (cliff bedore*)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: What is the voltage spec for thinnet?
Message-ID: <1991Feb5.131333.2047@isavax.isa.com>
Date: 5 Feb 91 13:13:33 GMT
References: <1991Jan30.155606.21529@dartvax.dartmouth.edu>
Reply-To: cliffb@isavax.isa.com (cliff bedore*)
Distribution: usa
Organization: ISA Inc. Arlington, VA
Lines: 41

In article <1991Jan30.155606.21529@dartvax.dartmouth.edu> wbc@moose.dartmouth.edu (Wayne B. Cripps) writes:
>
>
>What should the voltage be on thinnet? - I get readings of
>-1.8 to -2.0 volts and .2 to .3 volts - is this in the
>range?  is 1.2 volts ok?
>
>	Wayne

At the risk of stepping into something soft and gooey, I thought I'd put on
my engineers hat for a while and comment on this.  

First.  It will be very traffic dependent (assuming you're using a voltmeter
that does some averaging).

Having stated that and not knowing the details of an ethernet board, but
knowing something about transmission lines, we can get a wag of ranges for
the voltages.

The lines are 50 ohms and are terminated in 1/4 or 1/2 watt resistors. (Mine
is cause I did it myself and haven't had problems).

Power (watts) = voltage ^2 / resistance or

voltage = sqrt( power * resistance)

voltage = sqrt ( 1/4) * 50 ) or 3.5 volts. for 1/4 watt power

voltage = sqrt ( 1/2 * 50 ) or 5 volts for 1/2 watt power

These are for steady state DC levels.  Ethernet is 10mbit/sec square waves
+- distortion from the coax.  This means the the voltage measures via some
sort of averaging/rms/other meter would be something less (depending upon
the actual waveform, activity on the cable etc.  (This is true for thick or
thinnet. the impedance on the cable is the same)

This the voltages you measure will probably be in the range for correct
operation, but without looking at the waveform, you could have a short with
some DC voltage giving you the same result.

Cliff