Re: Appletalk/Phonenet box wiring (Re: ...phoneNet connection to Win2K?)

[russotto@wanda.pond.com (Matthew Russotto)]

In article <3b4c6986.212088632@news.mw.centurytel.net>,
Mister Boffo <mister_boffo@hotmail.com> wrote:
>
>It is possible that R3 and R4 don't "protect" RxD+/- but instead are
>there to make sure some power flows through the transformer from TxD+
>to TxD-. Without R3 and R4, all the output of TxD+/- could be sucked
>up by RxD+/- without doing anything in T1. 

I think it might be a bit more subtle than that.  The probable reason you
need a connection from your own transmitter to your own receiver is
for collision detection.  With a straight connection, your own signal
would overwhelm any signal from the bus, leaving you unable to detect
collisions. 

>Resistor R1 is still a bit of a question mark for me, but I do believe
>you can also construct a filter out of an inductor and a resistor. So
>in this case R1 and the "secondary" coil of T1 are working together to
>form a second noise filter for the Appletalk network bus, in addition
>to the R5/C1 noise filter...

I think R1 is probably to prevent external high frequency noise from inducing
a signal on the bus.  

What bothers me is the connection from the bus ground to serial
ground.  With that, if your computers are at different ground levels,
your Appletalk network can damage them.  What's the point of isolating
things with a transformer if you are going to directly connect
grounds?

Note that PhoneNet, being unshielded, doesn't have a bus ground.
-- 
Matthew T. Russotto                                russotto@pond.com
"Extremism in defense of liberty is no vice, and moderation in pursuit
of justice is no virtue."