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From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Req For Info - Ethernet Electrical Rules/Specs
Message-ID: <2230028@hprnd.HP.COM>
Date: 11 Oct 89 17:27:20 GMT
References: <188.2526de30@acci.com>
Organization: HP Roseville Networks Division
Lines: 38

re a reference for 802.3 cabling rules:

  The best reference is the source, ISO 8802-3, ANSI/IEEE Std 802.3,
  available from IEEE 345 East 47th Street, New York, NY 10017.  The
  media specs are in chapter 8 for thick and chapter 10 for thin. If
  you have an older copy of 802.3, it doesn't contain section 10 and
  you will need to have an old copy of the Supplements to 802.3 CSMA/CD
  or to buy a new copy. It is not friendly or tutorial. It tells the
  rules, it usually doesn't explain why.

re why using a pigtail sometimes works:

  802.3 MAUs (transceivers) present a high impedance to the bus to
  make it look as much like a continuous piece of 50 ohm transmission
  line as possible. Any shunt capacitance attached to the cable
  results in a reflection. The standard allows each MAU to cause
  no more reflection than a 6 pF capacitor. When you put on a pigtail
  you will be creating larger reflections than permitted.

  The reflections do two things. They reduce the power in the
  signal and they interfere with the signal as noise. Assume you 
  have a number of taps done with pigtails.  Depending on where
  you observe on the cable, where the transmitter is, where
  the taps are and what the data pattern is; the reflections may
 add or they may cancel each other. Also, some components in
 your system may be better than worst case allowing margin for
 the extra noise.

  So you may find things work fine or that most stations work
  fine, but A and B can't talk to each other. You can get some
  pretty flakey problems. It is not a good idea to do.

Same thing applies to 75 ohm cable. Depending on the exact transmit
level and collision detect of your MAUs, you may not see a problem.
You may have some MAUs that don't work and some that do. You can
get false collisions and you can get CRC/alignment errors.

Pat Thaler