Questions for Classical Electrodynamics.


Footnote to Questions for Classical Electrodynamics.

Ivor Catt. 23 March 2010





Yesterday, 22 March 2010, John Dore told me that USB has been using bidirectional signalling down one twisted pair for some years. This means that the section below, which I added to yesterday, means that I was waiting for something to happen which had already happened.

Ivor Catt


But today, 23 March 2010, on the phone with John again, I get the impression that USB has protocol for whether the next period of time will be occupied with signals in the one direction or in other direction. If this is true, then it makes my point forcefully. It will mean that lack of competence in electromagnetic theory worldwide has led to twice as many wires being used as is necessary (to achieve the same bit rate. But see “The Dream”, below. I want less wires for other reasons than bit rate).


If it is really true, as I have thought, that the concept of sending signals down one (pair of) wire(s) in both directions at the same time does not exist, then I had better give more information on the design of the necessary circuits. I would not be happy for someone else in the future to patent the idea. I shall be grateful if those reading this take a copy and annotate the date, as protection against exploitation by a patent later in time. I think that it would not be fair if someone read this idea of mine and then made a lot of money. I am not against profiting from patents. Sinclair gave me £500,000 in money and stock options for my patent on WSI.  Followed by The Kernel Machine.

Here is a broad brush analysis of the design.


Since signals are being sent in both directions, the 50 ohm line has to be terminated at both ends. This means that more power is required to send the signals.


At our end we have a drive circuit which delivers the voltage step into the signal line. The source is a current source which will result in a five vole amplitude across two 50 ohm resistors in parallel, the line terminations at both ends.


We also have a receiving circuit. We deliver the signal we are sending (5v or 0v) to this circuit, and put it on one side of a long tailed pair. The voltage across our 50 ohm terminating resistor is put on the other side of the long tailed pair. The result is that our signal is on both sides, and so cancels out, while the incoming signal will control the switching of the circuit.


Conceptually, it is better to think in terms of a very long connecting cable. Then having mastered the principles involved, gradually in your mind shorten the cable down to very short. The situation remains the same.


The dream, or objective, would be to interconnect two modules (for instance computer and printer) with one wire only. Unfortunately this seems impossible, because a signal travels down between two wires, not inside a wire. The whole irritating business of having the correct interconnecting cable and connectors would disappear. Two modules could be interconnnected using two wires and four crocodile (alligator) clips. The characteristic impedance, or spacing, between the wires in not critical, if one studies the formulae since a logarithmic term is involved.


Wireless is not the total answer, because of hacking and also lower bit rate is a carrier frequency has to be used.