Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site watcgl.UUCP
Path: utzoo!watmath!watcgl!dmmartindale
From: dmmartindale@watcgl.UUCP (Dave Martindale)
Newsgroups: net.lan
Subject: Re: Ethernet cable length query - (nf)
Message-ID: <2322@watcgl.UUCP>
Date: Tue, 27-Mar-84 14:03:10 EST
Article-I.D.: watcgl.2322
Posted: Tue Mar 27 14:03:10 1984
Date-Received: Wed, 28-Mar-84 01:06:44 EST
References: <6986@unc.UUCP>
Organization: U of Waterloo, Ontario
Lines: 23

I have heard that VAX 750's have 15volt power supplies which are barely
adequate for driving the Ethernet transceivers.  And, the transceivers
will have their own local filter capacitors for that incoming power,
so there will be a current surge when they are plugged in.  When DEC
installs a DEUNA and transceiver, a small panel is installed on the
back of the machine to provide the connection between the transceiver
cable and the short cable which goes the rest of the way into the
Unibus box to the transceiver.  This panel contains a current limiter
which prevents this large initial drain on the 15V supply.

If you are using someone else's Ethernet board which doesn't have this
current limiter, you are probably causing the 15V supply to drop so
low that the machine detects a power failure and reboots.

The problem with the DELNI Ethernet-without-the-cable is that it costs
approximately as much as 4 transceivers.  At Waterloo, we had to buy
Ethernet cable since not all of the machines to be connected were even
on the same floor.  There are 3 machines in the same room, but it is
cheaper to buy 3 transceivers than 1 DELNI and the one transceiver to
connect it to the main cable.  So the breakeven point where a DELNI
becomes worthwhile is about 4 machines in the same room if that is the
whole network, and 5 machines if they have to talk to a cable anyway.
Not nearly as attractive as one might have hoped.