Megaprocessor - built from individual transistors

Vincent Slyngstad v.slyngstad at
Thu Jun 25 19:15:55 CDT 2015

From: Doug Ingraham: Thursday, June 25, 2015 4:10 PM
> On Thu, Jun 25, 2015 at 10:22 AM, Vincent Slyngstad wrote:
>> The only other concern I've had during my thought experiments
>> along this line related again to the current/power involved.  The
>> device is likely to become difficult to cool if you achieve a 4X
>> volume reduction.
> The easy way to do this is to make one board with a small CPLD connected to
> all the pins and program it to match the logic of the card you are
> emulating.  That way you can have a couple of hundred boards made up for
> you at a reasonable cost that are all the same but ends up with different
> functionality.  Just stick a label on the handle to tell what the card was
> programmed as.

I've been thinking lately that whole Omnibus cards could be generalized 
that way.

> I don't think you would have heat issues unless you actually tried to use
> the original circuits.

I had assumed, since the subject contains "built from individual transistors", 
that something akin to the original straight-8 circuits was what was being 

Straight 8 signals typically run 8 mA when low and 10 mA when high, 
and that's from the -15V supply, per signal.  I estimate that's a few 
hundred watts for the whole thing.  Reducing the supply voltage to 
-5V would presumably cut the power (and hence the heat) to a third, 
by eliminating most of the waste heat from all the pull-downs.

> The Straight 8 logic uses pulses for some things instead of levels making
> it not really compatible with more modern logic.  At least not a direct
> replacement.  The later models would be better targets.  An I for example
> might be the best choice.

I like the 8/I too, though it's not "built from individual transistors".

> You can't shrink the size of the Front Panel because the switch spacing is
> already just a little narrow for people with large fingers.  It would be
> better to make it a little oversize in fact.   I don't have large hands and
> it would still be better for me if the spacing was slightly wider.

I completely agree, at least if you're actually going to use the darn 
thing a few hours at a time.  Some models of the PDP-11 did use much 
smaller (still available) switches, but fitted them with honking paddles 
to make them more comfortable to use.


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