Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!usc!trwind!johng
From: johng@trwind.UUCP (John Greene)
Newsgroups: comp.dcom.lans
Subject: Re: Req For Info - Ethernet Electrical Rules/Specs
Message-ID: <584@trwind.UUCP>
Date: 9 Oct 89 16:02:09 GMT
References: <188.2526de30@acci.com> <580@trwind.UUCP> <850@maxim.erbe.se>
Reply-To: johng@trwind.UUCP (John Greene)
Organization: TRW Information Networks Div
Lines: 37

In article <850@maxim.erbe.se> prc@erbe.se (Robert Claeson) writes:
>In article <580@trwind.UUCP> johng@trwind.UUCP (John Greene) writes:
>>In article <188.2526de30@acci.com> ta2@acci.com writes:
>
>>>2) Is is legal to run a pigtail of two or three feet from the
>>>   base of the BNC T to the Ethernet card?
>
>>This is a definite no-no.  Not only would you be radiating all over the place
>>(an FCC taboo) but you would cause an even more severe mis-match than if you
>>used a 75 ohm cable.  The impedance of the pigtail will depend on the thickness
>>of the conductors and the distance between them.  The cards would probably not
>>work with a pigtail an inch long let alone two or three feet.
>
>I've actually seen this, and it worked. The site put a thin ethernet segment
>in the walls and had the base end of the T connector going out of the wall.
>When there were a need to connect a workstation, a Mac or whatever to the
>network, they just connected a few meter long cable to the T connector and
>to the thin ethernet connector on the Ethernet board in the machine.
>
>Judging from prior knowledge, this just should't work, yet still it does.
>Can anyone explain this?

For one thing, this is not a "pigtail" as I took it to mean.  When we refer to
pigtails around here it is taking the coax and bringing it out to two separate
leads.  As to why the configuration that you descibe works...luck comes to mind.
One reason that I can think of off the top of my head would be that the 
Ethernet cards that are being attached provide an excellent match to the 
cable.  This will minimize the reflections going back to the "trunk" and
won't interfere with the signal there.  If the card does not provide a good
match, it can mess up the network without being active.  A good reason why
this should not be done.
>
-- 
John E. Greene    "People are just like frankfurters....You have to decide
                   if you're going to be a hot dog or just another wiener" DLR
TRW Information Networks Division 23800 Hawthorne Blvd, Torrance CA 90505
ARPA: johng@trwind.ind.TRW.COM  USENET: ..trwrb!trwind!johng