Relay computers

From: Adrian Vickers <>
Date: Fri Sep 24 05:55:19 2004

At 22:50 23/09/2004, you wrote:

>On Wed, 2004-09-22 at 11:17 -0700, Vintage Computer Festival wrote:
> > On Wed, 22 Sep 2004, Jules Richardson wrote:
> >
> > > Apologies if someone's mentioned this one before. Quite possibly the
> > > coolest gadget I've seen in a long time, though:
> > >
> > >
> >
> > Awesome. It would be great to get stuff like this exhibited at the VCF.
> >
> > The web page is pretty smart also. What terrific technical and design
> > work.
>Amazing, huh? If I read that right, it's 52 relays though (I don't know
>any German) which makes it sound rather like a 'simple' relay control
>system rather than an actual relay computer. I fired off an email to
>them to see if they'll let me have a nose at the wiring diagrams.

I did some back-of-a-fag-packet calculations (i.e. they might be completely
& utterly wrong, in which case I'd appreciate corrections) on a relay

Assume you want a Z80-type CPU. This has ~8k gates. Typically, it seems to
take 1 relay per input to implement any given gate. Now, I don't know how
many "x-input" gates there are in a Z80, so I'll assume that - on average -
it will require 3 relays/gate. Thus, we need ~24,000 relays to implement
the Z80.

If each relay needs, say, 25mA _at_ 6v to operate, then the peak current draw
of our R80 (as I shall call it) could be around 600A (I think). And that's
before we've added memory, i/o, etc.

As for the heat/noise - well, IMHO it's worth building it just to
experience that! Mind you, you'll need a lot of room: If you use 30mm by
13mm relays, then the board space you need is at least 9.36 square
metres... Still, if you assume that each board needs approx 40mm of space
incl. airflow room, then you should be able to fit the R80 into 2 400mm by
400mm by 2000mm cabinets (internal w/d/h)...

The relays I've been looking at typically quote around 25ms to operate
(either way), so I don't see how you could clock the R80 at anything faster
than around 40Hz; and you'd probably want to drop to 20Hz to be on the safe
side. The same relays quote a typical lifetime of 10e7 operations; so at
20Hz, your R80 should last a little under 139 continuous hours of operation
before relays started failing...

Creating a screen driver should be interesting....

Question: Wouldn't it be easier to implement an OR gate with no relays at
all (just two wires joining together)? Or would you need to use the relays
to keep the output voltage/amperage regulated?

Received on Fri Sep 24 2004 - 05:55:19 BST

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