Figure 1: Architecture of the ABC Proof-of-Concept Machine

Before beginning construction of the ABC, Atanasoff and Berry constructed a small 'breadboard' machine in 1939 to prove the feasibility of the memory and arithmetic concepts they intended to employ.

The proof-of-concept machine functionally consisted of two 25-bit registers, a 1-bit carry register and a 1-bit-wide arithmetic unit capable of addition and subtraction.

The registers were implemented using capacitors, one per bit, arranged radially on a revolving disc. Stationary contacts sensed the stored charge on the capacitors. These signals were sent through the electronics, and thence back to another contact to refresh or update the charge. Note this is the same principle (periodic refreshing of capacitive charge) employed today in dynamic RAM chips.

The arithmetic unit, or Add-Subtract Mechanism as it was called, was constructed with vacuum tubes and employed the principles of binary arithmetic, not counters. Addition and subtraction were performed in a bit-serial fashion with the 1-bit carry register holding the carry state from the processing of one bit till the next.

According to the diagram in the book "The First Electronic Computer", there were no input/output facilities on this machine. One can speculate that input may have been performed by briefly connecting appropriate points on the machine so as to insert a random number into the registers. Output may have been viewed on an external oscilloscope. After recording the injected random values as viewed on the scope, an arithmetic operation could be performed, and the result checked by again viewing the scope.

Nothing remains of the original proof-of-concept machine. In the early 1970s Atanasoff recreated some diagrams and a reconstruction was built for the ENIAC court case over patent rights to the concept of digital electronic computing.

A Simulation of the Proof-of-Concept Machine

	Run ABC Proof-of-Concept Simulation Requirements & Suggestions

An online simulation of the proof-of-concept machine is presented here. An oscilloscope is provided for output and pushbuttons for rough entry of bits.