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From: wnn@dsunx1.dsrd.ornl.gov (W. N. Naegeli)
Newsgroups: comp.protocols.appletalk
Subject: Re:  Need help (actually confirmation) with extended Phonenet wiring
Message-ID: <8904111241.AA26349@dsunx1.DSRD.ORNL.GOV>
Date: 11 Apr 89 12:41:59 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
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Jeff White ponders whether it would be beneficial to terminate each branch
of a phone net with a resistor tuned to achieve a network impedance of 60
ohms.
I don't have an anser to this problem, but it seems to me that this is
exactly the theory between Nuvotechs TurboNet ST (self-terminating)
connectors. Nuvotech claims that their ST connectors "add just the right
amount" of resistance and "adjust this dynamically" as other nodes are
added or taken off the network.  We have an extensive phone net of very
complex topology (multiple stars, some with small trees and daisy chains
attached).  In addition to 120 ohm resistors at the extreme ends of the trunk
that connects the stars we are using TurboNet STs at most of the longer 
branches and for all noisy devices such as laser printers.
Our current effective network length (counting branch lenght twice and
26-gauge flat extension cables four times) is about 4000'.  There are 46
drops and 18 devices connected at present. As one would expect with such a
crazy topology we have been experiencing some problems for the last few days
since the last 12 drops were installed (some nodes can't see all other nodes).
However, I expect these problems to vanish when the TurboBridge arrives
that we ordered in anticipation of this situation and which will replace
the Tops Repeater that is located at the middle of the trunk.
In the meantime I can give preferrential treatment to some nodes by moving
a couple of 120 ohm resistors among the ends of various branches. This 
seems to indicate to me that signal strength and network impedance, which stay
the same are not as critical in our case as reflections and noise.
That's why I expect the bridge to take care of the problems. The repeater
also amplifies noise.
I have not been very systematic in my termination trials, but my gut feeling
is that the self-terminating connectors are beneficial in general, but that
placing a 120 ohm resistor and a standard (non-terminating) connector at the
ends of the trunk works better than using TurboNet STs at these locations.
Another indication that noise is a principal cause of the problems is the
fact that it makes a difference whether certain devices are on or off.
Even when thay are idle, they seem to put out some noise. We need to be
particularly careful that nobody switches the NEC SilentWriter LC-890 to
parallel mode without first disconnecting it from the network, otherwise
it brings down half of the network.
NuvoTech does not reveal what they are doing inside their ST connectors
and within what range it adds resistance. I'd be interested to find out.
Has anyone taken an ST apart?
Wolfgang N. Naegeli
Oak Ridge National Laboratory
wnn@dsunx1.dsrd.ornl.gov (128.219.96.46)
(615) 574-6143