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Eckert and Mauchly: Electronic Numerical Integrator And Computer (ENIAC), 1949

The Decade Counter

In ENIAC, vacuum tubes were used as on-off switches and were instrumental in the success of the counting circuits which Eckert developed. ENIAC's counting circuits contained ten flip-flops, and each flip-flop was made up of two vacuum tubes. Eckert wired the flip flops so that only one tube could be on at a time. If vacuum tube A was on and the next pulse came in, the flip-flop flipped to vacuum tube B on, A off. Each electronic pulse resulted in a "flip-flop" of electricity, so that the flip-flop that remained on at the end represented the final sum (for example, if ENIAC wanted to add three and five, only flip-flop eight would be on at the end of the calculation). Each counter thus had twenty vacuum tubes to represent each ten-digit number, and eight other vacuum tubes that represented plus or minus signs and other controls. Eckert's decade counter provided the crucial technology for turning pulses into numbers.

Moore School army liaison John Brainerd and ENIAC engineer Kite Sharpless co-authored an article on the ENIAC which was published in the February 1948 issue of Electrical Engineering. The article extensively details the structure and function of the ENIAC and is an excellent source of technical description. You can access the piece in full by clicking on the thumbnails at right.