Path: utzoo!attcan!uunet!lll-winken!elroy.jpl.nasa.gov!sdd.hp.com!hplabs!hpda!hpcupt1!hprnd!pat
From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Wiring weirdness on 3B1 STARLAN (long)
Message-ID: <2230102@hprnd.HP.COM>
Date: 19 Jul 90 16:44:41 GMT
References: <1318@mtek.uucp>
Organization: HP Roseville Networks Division
Lines: 45


A couple of questions:

ATT has marketed Ethernet over twisted-pair products as StarLAN 10.
Since you say only StarLAN, I assume you are talking about 1 Mbit
StarLAN products.  Is that correct?  One reason I am confused is
because you mention 400 feet as the distance limit when 1 Mbit
can support 250 m.

Is the wire you were used from the jack to the 3B1 truely twisted
pair or is it flat?  

In what way does it not work?  Do you get collisions, CRC errors,
or does the data just not get throught?  Do both directions of
communication fail to work or only one.

You may be to the point where it will be hard to pin down the
cause of the problem without testing the wire or by testing only
DC parameters of the wire.


On terminating resistors:

AT&T and some other manufacturers produced a bus version of StarLAN
(the standard doesn't cover a bus, only a point-to-point link).
When using a bus version, terminating resistors are needed.  I believe
that in the AT&T products those resistors are actually built into the
interface cards and are automatically connected when needed (based
on which jacks have connectors plugged in) so as a user you would
not necessarily be aware of them.  (Bus products generally have
two RJ-45 jacks, so the bus goes through the card.)  Some other
bus type products require a terminatting resistor module to be plugged
into the unused RJ-45 jack at each end of the bus.  

Since you don't appear to be using a hub, I assume you are using
the AT&T cards in bus fashion.  Also because you mention a distance
limit of 400 feet.  1BASE5 (the name of the standard covering 
StarLAN) allows a distance of 250 m (about 800 feet) from the card 
to the hub.  A bus version used without a hub would cut that
roughly in half.

In non-bus versions, the transmitter and receiver terminate the
line.

Pat Thaler