🚀 go-pugleaf

  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.
  Farallon was nice enough to send me a copy of their Phonenet installation
book, which describes some various wiring configurations.  Before I go on,
I should mention that right now, I am actually using Apple Localtalk 
transceivers, but using unshielded wire.  At first I had about 4 or 5 branches
connected, some with 2 or 3 devices on each, and there was some qualitative
problems (Tops workstations kept on losing contact with their servers).  The
reason, I suspected, was that all the Localtalk terminating resistors were 
connected.  So what I did was to plu some dummy connectors in the 2nd port,
which opened up the resistor.  I believe I kept 2 or 3 TR (term. rest.) in
the circuit).  That seemed to improve things until I started to add more
stations, and performance seems like it might be getting bad again, this time
probably due to reflections.
  Back to the Farallon book, after reading through it, it seemed as terminating
a line to absord reflections was a definite must.  Whether the wiring was a
daisy chain, bus, or active or passive star, they always stressed using the
120ohm TR.  I accepted this as being realistic.  What I needed to do was 
figure out what resistance to use, if I wanted to put a TR at the end of
every branch (remember, I'm looking at 7-10 branches, more than double what
they recommend).
  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.  
  What I am thinking about doing is expanding this thinking to 8 or 10 
branches, or TR's.  Instead of using 120 ohm resistors at two ends, would
there be a problem with using 600 ohm resistors at each of my 10 branches?
Actually, what I'm trying to find out as if it seems correct to just balance
the resistance of the TR depending on how many branches there were, with
the object being to keep the overall impedance around 60 ohms or so?  Right
now I have 6 branches, and since I can forsee adding probably 3 in the short
future, I'm leaning towards 470 ohm TR's.  That would give my circuit an
impedance of about 78 ohms now (n=6) and about 52 ohms later (n=9), both pretty
close to the ideal of 60 ohms.
  Assuming this theory works, would it be better to be above or below that
ideal value?  For example, it I switched to 390 ohm TR's, at n=6 my impedance
would be 65 ohms, and at n=9 it would be 43 ohms.  Which range would be 
better?  Also, would it be better to keep all the TR's the same, or fine 
tune as I go along?  For example, if I start out with 5 nodes with 300 ohm TR's
(net imp=60 ohms), if I add a 6th node, should I use a larger TR, so the overall
impedance doesn't fall as rapidly?
  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.

						Jeff White
						Univ of Penn - CETS
						jeff@eniac.seas.upenn.edu