PDP11/74 - test-drb@ccmp.vtda.org

15 Dec 2003

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Message: 16
Date: Sun, 14 Dec 2003 18:52:21 -0800 (PST)
From: "Eric Smith" &lt;eric(a)brouhaha.com&gt;
Subject: RE: Building a PDP-11 for the first time
To: "General Discussion: On-Topic and Off-Topic Posts"
	&lt;cctalk(a)classiccmp.org&gt;
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	&lt;33073.64.169.63.74.1071456741.squirrel(a)ruckus.brouhaha.com&gt;
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Antonio wrote:

   I
don't know what the performance figures were for the
 11/74 either. However, the beast was deemed to be too
 hard to maintain. Too many cables, too much downtime.     

That was one of the rumors, but I've never believed it.  There's no fundamental
reason why it should have been any harder to maintain than any other large computer.  I
think this was just a feeble attempt by DEC management to come up with an excuse to give
customers, when they didn't want to admit the real reason the product was cancelled.

Yes, it needed more memory cables than any other PDP-11, but it's
utterly ridiculous to think that would be a serious problem.  We're only talking about
68 memory cables per CPU for a maximally configured PDP-11/74, compared to 16 for a
maximally configured PDP-11/70.
When was the last time you had a PDP-11/70 memory cable go bad?

The memory cabling was in fact comparable in magnitude to that of a large VAX 11/782
configuration.

Gimme a break.  The MK memory cables were often the cause of multiple 
problems and the multiport version set up on a bunch of conjoined 
11/70-type boxes
would've been a nightmare.  XXDP+ would've had to be loaded into a 
shared memory partition and it would've been a large PITA for the 
systems manager
to have to do that while it was running.

They could've required a preloaded diag monitor that could've been 
switched to the down box, though to get around this.

Anyway, the 11/782's were large pains only when the SBA interconnects 
went bad
which was a fairly rare occurrance when the machine was fully 
installed.  The SBI
cables were about 4 inches long... the 12-18 inch ones were used between 
cabinets.

How many feet of MK cable would that have been!

The MK cables were often nicked, cut or just plain and could've been 
sliced by tiles
in multcpu configs... etc.

I used 1/4 of an 11/74 at DEC Princeton.   A nice pretty front panel and 
I'd have loved to work on one full-up.

But maintaining it would've been a PITA.

BIll
pechter(a)monmouth.com