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From: jstewart@ncs.dnd.ca (John Stewart)
Newsgroups: comp.dcom.lans
Subject: Re: SQE test on transceivers
Keywords: "enable or disable"
Message-ID: <1990Oct3.204717.24401@ncs.dnd.ca>
Date: 3 Oct 90 20:47:17 GMT
References: <1990Oct3.191251.26769@watserv1.waterloo.edu>
Distribution: na
Organization: Dept of National Defence
Lines: 46

In article <1990Oct3.191251.26769@watserv1.waterloo.edu> nbirkman@watserv1.waterloo.edu (Nancy Birkmann - DCS) writes:
>

This is a recurring topic. I am sure that you will receive
many answers, so I'll just address the points, and I'll
leave the philosophical disucssions to others. :-)

>
>I  read a statement that the SQE signal is not allowed for
>transceivers that connect to 802.3 repeaters.
>Why?

Because SQE and Collision detect are two names for the same
set of wires. The meaning of the wires  is determined by the
timeframe relative to the transmission of a packet; see the 802.3
documentation. Most repeaters do not buffer, so don't really know
the wheres and whens of the beginning and end of packets. Remember
that a repeater repeats what it sees on one segment to all others.
If it sees the SQE as a collision, then it will jam all other segments
to propagate that collision. Ditto for connecting a repeater up to
a fanout box where the fanout box is connected via an SQE enabled
tranceiver.
 
>
>Also, many transceiver manufacturers (e.g. Allied
>Telesis, Cabletron, Interlan...) provide an option of
>either providing this signal or turning it off.
>Under what circumstances should the signal be turned off?

I leave it off, unless the equipment really needs SQE. If it
does, and one is not present, you will receive a message indicating
such. You will probably receive many different opinions.

>Most of the transceivers in our system to date have been
>Digital H4000s where these options were not available.

The H4000's are field retrofitable to enable/disable SQE/Heartbeat
by cutting a capacitor lead inside them. Also, I think that the
H4005 or the H4000-BA (with the lead cut from the factory)
are the non-SQE'd tranceivers, while the standard H4000 is the
one with the SQE. It's been a while since I have been around
a lot of DEC stuff, so treat the above paragraph with due caution.

John Stewart
DREnet Coordinator
jstewart@ncs.dnd.ca