Signals

Signals

As you can see from the illustration below, an electron head-tail pair (which we’ll call an ‘electron’) moves along a row of copper cells (which we’ll call a ‘wire’) at the rate of one cell per generation.

electron head-tail pair moving on copper wire

Wires give us a way of transporting a sequence of electrons from one point to another. The pattern of electrons is preserved as they move along a wire and so, if we can represent data using these patterns, we can use the wires to carry data from one place to another.

A simple approach is to divide a wire into segments of equal length - call this length n. If there is an electron at the start of the segment, it represents a binary ‘1’ bit; if the segment is empty, it represents a ‘0’ bit. As long as the two ends of a piece of wire are synchronised (so that the receiver knows when a segment is about to start), we can communicate data like this.

What value of n shall we use? A segment has to be big enough to hold a head and a tail, and so n must be at least 2. However, as you can easily verify, it’s necessary to have at least one gap between successive electrons, and so in fact n must be at least 3. We say that a system using a particular value of n is employing n-micron technology.

As Michael Greene has shown, it is possible to implement all the logic and other functions we need for the computer in 3-micron technology. However, except for the computer’s display (which you can see on the index page) our implementation is in the less aggressive 6-micron technology as this makes the design simpler and probably smaller. The original Scientific American article suggested using 13-micron technology.

The picture below shows some 6-micron signals.

some 6-micron signals

The top wire is carrying a steady stream of zeros; the middle wire, a steady stream of ones; and the bottom wire, the sequence ‘1110’ repeating forever. The middle wire also shows how a signal can be split into two copies.

Another approach to representing data on a wire is to divide it into segments of length n and represent a binary ‘1’ by an electron at the start of the segment and a ‘0’ by an electron one cell further back: the logic state is represented by the phase of the electron relative to a fixed clock. This strategy has some attractive features, but we did not consider it until design of the the computer was well under way.

Next, the diode.

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This page most recently updated Mon 16 Jan 11:10:09 GMT 2017
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