that's an AUI terminator.  you'd plug this into a delni's
uplink port, or to a host, if you wanted to see "carrier".

it's particularly useful on a delni's uplink port, since
the delni's "standalone" switch causes it to intentionally
jabber at the end of each packet ("ethernet heartbeat")
and if you build a tree of delni's and want your root to
not kill the whole net with heartbeat jabber, you need a
terminator in the root's uplink port.

ethernet heartbeat was added in the D-I-X committee.  xerox
knew full well that silence equalled assent and that without
a higher level feedback protocol like xns or tcp or tp4 or
even decnet, there was no way to build a reliable network.
but the D-I-X committee thought they knew better, thus,
heartbeat.

> Date: Tue, 01 Jan 2002 12:43:59 -0500
> From: Peter Hufnagel <CaptnZilog@aol.com>
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> To: port-vax@netbsd.org
> Subject: Terminator of some kind?
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> 
> I have 5 DEC...umm, terminators I guess, of some
> sort... 15-pin 'D'-shell type connector, male, with
> only 6 pins in it, and a green LED on the outside,
> DEC P/N 12-22196-01.
> 
> Anyone know what they are?  
> Anyone have a use for them?
> 
> Pete