On 5/11/2010 8:56 AM, Invisible wrote:

> Perhaps you would have enjoyed a visit to Stoney Stratford telephone
> exchange. It's a sprawling building, filled with rack after rack after
> rack of cases, each case filled with dozens of relays. And they're not
> in cases or anything, just "naked", so you can see (and if you desire,
> move) the working components. There is also several tonnes of wire
> overhead feeding this equipment. And all of it is as silent as death.
> It's like a museum or something.

Some time ago I was reading a Wikipedia entry about some of the relays 
in a central office. Someone described the sound as deafening back when 
everything was electromechanical. Especially on heavy phone traffic days.


> Myself, I visualised a computer made of pressurised water fed through a
> series of simple mechanical valves. Sadly, I fear that for reliable
> operation, you would need inside water pressure. And if you wanted a
> clock speed of more than about 0.02 Hz, you would have to use steel
> piping and pyrotechnics to sustain the necessary pressure! It would be a
> very "kinetic" experience though.

Hmm. Water hammer. I can't imagine it working purely on water pressure 
alone, though. I keep thinking solenoids to control the valves. Would 
that be cheating?

> Also... when I tried to build my own machine out of 7400s, I quickly
> discovered that the gates don't appear to function as their truth table
> indicates that they should. (!) I also looked into playing with FPGAs,
> but the cost is prohibitive. (And, knowing me, I'd just make something
> that doesn't even work, and then spend months trying to find out why!)
> Plus, Xilinix (?) have a free simulation tool available, and it's just
> painful to use. I dred to think what actually synthesizing with it would
> be like...

They do. If, however you have the gate connected to a switch, and the 
switch opens the circuit, you'd better have a pull-down resistor, or the 
gate will float. TTL is rather forgiving of this, the input will float 
high (e.g. it will be interpreted as a 1) CMOS, on the other hand, can 
potentially self-destruct if an input is left floating ;)

>> The TTL version should be fun, too. I plan to have lots of LEDs to
>> show what's happening inside the machine.
>
> In solomn truth, it's probably simpler and easier to code a small peice
> of JavaScript that controls a little Flash animation on a computer
> screen. But there's something impressive about being able to pick up a
> physical object in your hands and see that there really are no tricks...

Yes, but, as you say ... there is something impressive about a physical 
object. :)
-- 
~Mike

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