Path: utzoo!attcan!uunet!ginosko!gem.mps.ohio-state.edu!tut.cis.ohio-state.edu!ucbvax!amdcad!rpw3
From: rpw3@amdcad.AMD.COM (Rob Warnock)
Newsgroups: comp.dcom.lans
Subject: Re: Req For Info - Ethernet Electrical Rules/Specs
Message-ID: <27683@amdcad.AMD.COM>
Date: 8 Oct 89 00:11:38 GMT
References: <188.2526de30@acci.com> <580@trwind.UUCP>
Reply-To: rpw3@amdcad.UUCP (Rob Warnock)
Organization: [Consultant] San Mateo, CA
Lines: 52

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.  The problem may not be noticed with a small number
| of nodes and short cable lengths...
+---------------

Actually, if you also use 75-ohm terminators at the cable ends [which is
needed to match the 75-cable, no?] , you'll notice trouble even with *tiny*
cables. You see, Ethernet uses a D.C. voltage-sensing method for collision
detection. When transmitting, Ethernet transceivers inject a known current
(*not* voltage) into a known 25-ohm impdedance [the parallel connection of
"two" 50-ohm cables, namely, the cable to each side of the transceiver],
which creates a voltage per E = I * R. If two stations transmit at once,
the currents add, and thus so do the voltages, and the collision is detected
by each station measuring that the local *voltage* is more than can be
accounted for by the locally-injected *current*.

Now if all cables were tiny and all transceiver's transmitters exactly
the same, the best voltage threshold to use for detecting collisions
would be exactly between the voltages of one transmitter and two concurrent
transmitters, or 1.5 times once transmitter. But cables *can* be long, and
the other guy's transceiver *can* be a little below par, so a better place
to set the threshold is a little lower, say 1.25 or 1.2 times your own
nominal output voltage (and that's in fact what commercial transceivers do).

O.K.  So you decide to use 75-ohm cable, 'cause it's cheap and you can get
it at Radio Shack or whatever. And you terminate the ends with 75-ohm
resistors to avoid reflections. And the first time any transceiver on
the cable starts transmitting, it'll see a local voltage of 1.5 times
what it expects (75 / 50 = 1.5), and will scream "Collision!" over and
over. Oops!

Stick to the 50-ohm cable. [And for long runs, I mean the *50* ohms of such
cable as RG58/AU or RG58/CU, not *53* ohms of RG58/U.] And try to get all the
cable for one Ethernet from the same spool, or at least the same lot number.

[Note that you can use *any* kind of cable, or even bare wire, on a *really*
short cable -- say, one foot -- as long as you have two 50-ohm (or one 25-ohm)
resistive terminators. Useful to know for that late-night quick hack on a
benchtop.]


Rob Warnock
Systems Architecture Consultant

UUCP:	  {amdcad,sun}!redwood!rpw3
DDD:	  (415)572-2607
USPS:	  627 26th Ave, San Mateo, CA  94403