Several people have asked what kind of logic (as in DTL, TTL
etc) I'm using, and why.
Until recently I generally replied "kind of TTL", afterall I was
using transistors. When I first wrote this page I thought a bit
more about it and changed my mind to answer "semi CMOS". A friend
has since pointed out that actually this is NMOS logic and is
described here on Wikipedia
Most definitions of TTL talk of using multi-emitter transistors. I
presume these are easy enough for the chip makers to build but
I've never seen any as discretes.
The whole project started with the objective of learning about
transistors so the answer was always going to be some kind of
transistor logic but I did look around at other options once I'd
decided I was going to build a processor. In my case DTL doesn't
have any real attraction. It seems to use much the same number (if
not more) of components as transistor based logic so size is much
the same. Marginally cheaper but trivially so. A biggish downside
in my view is that the fanout is quite low so you need to keep
track of that and insert buffers as needed.
Looking at CMOS gates I found circuits such as that below for a
If you're approaching this circuit from the viewpoint of building
some hundreds of them by hand the standout feature is that it
implements the logic twice. You have one version yanking the
output low when "0" and, a complementary version pulling the
output high when "1". Makes sense for a chip maker but for me it's
a definite no. So I chopped off the top half and replaced it with
a resistor. Doing this we end up with NMOS logic. The C in CMOS
stands for complementary i.e. it uses both P and N transistors.
Using just N transistors makes it NMOS logic which was used a
One consequence of this is that that it's much less power
efficient. The proper circuit only draws significant current when
switching (when both the top and bottom circuits might both be
"on" concurrently as they transition state) whilst my version
draws current all the time it is in the "0" state. However this is
irrelevant in the case of the Megaprocessor because we're burning
much much more power in the LEDs.
One of the benefits of using FETs is that the input impedance is
extremely high so I get a huge fanout which means I can be relaxed
on that front.
Actually my first thought was to use BJTs rather than FETs. As I
mentioned at the start my initial motivation was to learn about
transistors, and to my mind BJTs are more "proper" than are FETs.
And here is a picture of the first logic gates I built (using
2N3904). First LEDs as well! I think the pair on the left was my
first AND, and the next two pairs were my first XOR. It was a
I switched from the BJT (2N3904) to the FET (2N7000) when thinking
about how this was going to scale. For this project resistors
"cost" as much as transistors. They take as much space, and are
more fiddly to insert in a board and solder. With a FET
circuit you don't need the bias resistor that you need for a BJT
based circuit. So you can roughly half the component count by
switching to FETs. Also the fanout of the BJT logic gate is less.