Path: utzoo!attcan!uunet!munnari!gwydir!gara!dheap
From: dheap@gara.une.oz (Dave Heap PSYS)
Newsgroups: comp.protocols.appletalk
Subject: Re: Need help (actually confirmation) with extended Phonenet wiring
Message-ID: <649@gara.une.oz>
Date: 14 Apr 89 02:38:10 GMT
References: <9750@netnews.upenn.edu>
Reply-To: dheap@gara.UUCP (Dave Heap PSYS)
Organization: Uni. of New England, Armidale, NSW.
Lines: 52

In article <9750@netnews.upenn.edu> jeff@eniac.seas.upenn.edu (Jeffrey M White) writes:
>
>  I'm in the process of setting up a larger than recommended passive star
>7-10 branches instead of 3-4) using Phonenet type connectors and wiring, and
>need some help with the final installation.  (for those who may suggest it,
>I know about Farallon's Star Controller, but the price is too high (> $1000),
>and this installation is sort of a test installation anyway).
>  My problem has to do with the terminating resistors.  For a long time, I
>was confused as to whether the terminating resistors were:
>  1. To absord reflections - in which case you wanted one at the end of every
>     line, or 
>  2. To keep a constant impedance across the line - in which case you would
>     only want several of them.

	The answer is (1), to absorb reflections. They work best when matched
to the characteristic impedance of the line. Whether twisted telephone cable is
really a 120ohm transmission line at the frequencies we are talking about is
somewhat doubtful, however. (For an explanation of characteristic impedance
& termination practices look up a textbook on transmission line theory -
too involved to explain satisfactorily here).
	However, once you start adding branches you have problems. They should
be terminated to reduce reflections, but each branch then increases loading
on the system. (Even unterminated branches contribute to loading.) Farallon's
recommendation of terminating the four longest branches is a compromise between
reflection elimination & excessive load ( & therefore ability to drive the net).
You'll find that they limit the number of recommended nodes on this sort of
topology because of the drive problem (each node also loads the line).

	In summary, it's all a matter of compromise & whether you gain by
putting more higher impedance (& hence less effective terminators) or less
correct terminators is best determined empirically - if it works, fine.

>  It turns out that if you follow their guidelines exactly, you will always
>have at least 2 but never more than 4 TR's in the circuit.  Since everything
>on AppleTalk is in parallel, and the resistors as 120 ohms, that equates to
>between 60 ohms (for 2 TR's) and 30 ohms (for 4 TR's) impedance across the
>line, with closer to 60 ohms probably being better.  

	Not quite true, see above.

>  If anyone has done alything like this, I would appreciate hearing from you.
>Unless I hear that it WON'T work, I will probably go ahead and try it.

	We've set up a passive star in excess of Farallon's recommendations
& got away with it. It's a case of try it & see, keeping in mind the compromises
you are making.

-- 
Dave Heap              	         ACSNET: dheap@gara.une.oz                  
Psychology Department,           UUCP: ...!uunet!munnari!gara.une.oz!dheap  
University of New England,       ARPA: dheap%gara.une.oz@uunet.uu.net       
Armidale NSW 2351, Australia