Spacewar! (was Re: Laser display...)

From: Jim Beacon <>
Date: Sun Feb 20 16:48:04 2005

At the risk of telling everyone things they know.....

What we call a digital circuit is one that is designed to have two output
states (sometimes three, before anyone says it!), and these days we are used
to seeing them in small black packages. Going back in history, they were
built of discrete components, either semiconductor or valve (tube).

If you look at these circuits, you will find they are amplifiers that are
driven in to clipping - the simplest example is the inverter, which is a a
gate that gives the oposite ouput to its input. This can also be said of a
simple one transistor (or valve) common emiter (common cathode) amplifier.
The difference is the applied bias, and the applied input voltage swing. In
an amplifier you choose the base (grid) bias to place the operating
characteristic in the center of its linear part, and limit the input to a
small range of values, in an inverter, you bias the input to cut-off, then
apply sufficient input to drive the stage into saturation. Without doing the
maths on the circuit, you can't tell which!

The point of this rambling, as some have pointed out before, is that the
circuit is the same, it is the mode of operation that differs. When you
start looking at valve (tube) logic, it is important to remember these
points, tube logic is very critical of input voltage swing and bias
conditions (hands up all those who have struggled with the triggering
circuits in a Tektronix 500 series scope), early transistor logic is
similar, but it starts to resemble the circuits we are familiar with now.

You can also get some useful digital functions from analogue circuits -
Tektronix time mark generators use a non-retriggerable monostable to achieve
a divide -by-10 function with 4 tube elements (two of those are diodes) -
how many transistors would we use to build this in bistables? I recently
built a protoype divider using a single thyratron to achieve a similar
division ratio with a total component count of 6 - though you could consider
the thyratron a true digital device, the details are here

Finally, we all worry about the linear part of digital circuits in
computers - that little section of the timing diagram, which shows a high /
low or low / high transition, is the linear part of the gates operation -
risetime is actually the gate doing a linear transition between states (and
yes, you can build useable audio amps out of 4000 series inverters).


----- Original Message -----
From: "Vintage Computer Festival" <>
To: "General Discussion: On-Topic and Off-Topic Posts"
Sent: Sunday, February 20, 2005 10:08 PM
Subject: Re: Spacewar! (was Re: Laser display...)

> On Sun, 20 Feb 2005, Eric Smith wrote:
> > > The Odyssey is just a non-digital game unit that is designed with
> > > non-digital electronics.
> >
> > I disagree. It'd definitely partly digital.
> Okay, in what way(s)?
> --
> Sellam Ismail Vintage Computer
> --------------------------------------------------------------------------
> International Man of Intrigue and Danger
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Received on Sun Feb 20 2005 - 16:48:04 GMT

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