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Re: netbooting Maxine (MOP, TFTP) (fwd)

To: linux-mips@fnet.fr
Subject: Re: netbooting Maxine (MOP, TFTP) (fwd)
From: Richard van den Berg <R.vandenBerg@inter.NL.net>
Date: Sat, 27 May 2000 16:34:57 +0200 (MET DST)

---------- Forwarded message ----------
Date: 13 Dec 99 01:44:49 GMT
From: Toru Nishimura <nisimura@itc.aist-nara.ac.jp>
Newsgroups: muc.lists.netbsd.port.pmax
Subject: Re: netbooting Maxine (MOP, TFTP)

>> Possibly.  Quality AUI<->UTP MAU (media access unit) has a user
>> selectable 'SQE' switch.  The switch must be ON for 'station' (your
>> MAXINE case), OFF for 'signal repeating device' like repeater or
>> bridge.
>
> That did the trick!!! Off all the units i could choose of... this seemed
> just the one with the switch off ! 
>
> Switching it on... make it netboot!

Here goes the explanation.

DECstation ROM monitor (Digital calls it Rom Executive 'REX') uses SQE
to make sure the ethernet link is working properly for every
transmition.  SQE works as if 'acknowledge'.  If SQE is not received
by DECstation after transmittion, REX thinks network is choked.
DECstations require SQE signal (as SQE is designed). 

SQE is a 'local protocol' in gray 'drop cable' between station and
transceiver.  Transceiver is a device directly attached with a yellow
coax (10base5) cable.  In these days, drop cable has gone and adaptor
units are widely used to hook old AUI port to BNC or UTP cable.

Now why the SQE has to be user-selectable?  That's because in some
cases AUI port is used for signal repeating or 'port cascading'
purpose.

Let's say you have 10baseT UTP 'hub' device which happens to have AUI
port for 'up link' or 'neighbour link'.  You might hook an AUI->BNC
adaptor to it.  In that case, SQE must be OFF.  Or, every transmition
would propergate SQE condition to all of UTP ports.  Now users would
complain 'this 10baseT network is slow and congested all the time...'
Someone careful might notice the circumstance looking at 'collision'
LED is blinking whenever even single station is talking.

The story above is pretty common, and worth remembering.  It's funny
that in some cases people would try to resolve the 'congestion'
replacing the 'dumb hub' with a 'smart switching hub'.  This would
result in partial success and waste of money.

Thoru Nishimura
Nara Institute of Science and Technology



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