Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!rutgers!bellcore-2!envy!karn
From: karn@envy.bellcore.com (Phil R. Karn)
Newsgroups: comp.dcom.lans
Subject: Re: 50-75 ohm impedance matching
Message-ID: <1990Jul20.235220.22618@bellcore-2.bellcore.com>
Date: 20 Jul 90 23:52:20 GMT
References: <1990Jul13.210239.17750@ux1.cso.uiuc.edu> <8601@uhccux.uhcc.Hawaii.Edu> <28221@cc.usu.edu> <8611@uhccux.uhcc.Hawaii.Edu> <64296@sgi.sgi.com>
Sender: usenet@bellcore-2.bellcore.com (Poster of News)
Reply-To: karn@thumper.bellcore.com
Organization: Packet Communications Research Group (Bellcore)
Lines: 19


In article <64296@sgi.sgi.com>, rpw3@rigden.wpd.sgi.com (Rob Warnock) writes:
|> Nope, the D.C. signal is *not* necessarily "short". Consider a long stream
|> of back-to-back long packets, and note that the gaps between the packets do
|> not allow time for the D.C. balance to "relax". There is no limit to the
|> maximum length of the D.C. "pulse" as seen by the autotransformer.

Quite true. In fact, I found a cute use for this DC component.
Because the Ethernet modulation is Manchester, and because the signal
swings between 0 and -2V, by putting a 0-1V DC analog voltmeter across
the cable you get a very simple and effective real time network loading
monitor! 0 volts corresponds to an idle network, and 1 volt
corresponds to a saturated network. Of course, the accuracy may suffer
across a long cable, but hey...

Those who used to watch the analog CPU utilization meters on the old
IBM 370/168 consoles should appreciate this little hack.

Phil