A detailed description of the computer designs by Konrad Zuse can be found in the book by Prof. Raul. Rojas, Die Rechenmaschinen von Konrad Zuse, Springer Verlag, Berlin, ISBN 3-540-63461-4, 1998 (in German). The book also documents the unbelievable story of Zuse's patent application for the digital computer being rejected by the German patent office due to reasons of "niedrige Erfindungshöhe" (not worth patenting)...
The main feature of the adder circuit is the use of a dual-rail encoded carry-signal. That is, both the positive and the negated polarity of the carry-signal are inputs to the circuit, and both the positive and the negated polarity of the resulting carry-output are propagated by the adder. Note that only two relays (with four crossover switches each) are required for a one-bit full-adder.
Note that the carry-signal is propagated instantly after the input values A and B and the (dual-rail encoded) Carry input are applied to the circuit. Therefore, an n-bit adder is almost as fast as a 1-bit adder, because the propagation of the electrical signals is much faster than the switching time of the relays.
One additional relay, which is not included in the schematics, will be necessary to generate the inverted CarryInputNeg signal from the CarryIn signal for the first level adder.
Click the input switches or type the ('a','b','c' and 'd') bindkeys to control the circuit and watch the behavior. For correct operation of the circuit, the CarryIn and CarryInNeg inputs must have opposite polarity!
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