Eckert and Mauchly were cautious when they set out to work on ENIAC. They had a basic design, but they admittedly had not worked out exactly how they were going to realize their plans. They had decided that ENIAC would have three main parts. First, there would be self-contained machines built to handle mathematical operations: these machines would perform addition, multiplication, division and square roots. Second, they devised memory units to store numbers and instructions. Third, ENIAC required a master programming unit to plug mathematical orders into the machine and to manage the electronic pulses it used to represent numbers and mathematical operations. In addition to these three main parts, ENIAC called for peripheral units that initiated computation and kept calculation synchronized. Though computers have undergone myriad changes since ENIAC, the design outlined by Mauchly and Eckert at its inception remains the basic structure of computers even today.
Mauchly and Eckert wired the units of ENIAC together so that the numbers could be shared from one unit to the next and so that instructions could be sent over electronic cables. The two scientists bundled these cables together and arranged them in phone-like trunks they called digit trays. Each digit tray had eleven wires so that it could carry a ten-digit number and a plus or minus sign. The digit trays were permanently wired throughout ENIAC.
Mauchly and Eckert designed moveable trays called program trays, which could also carry ten-digit numbers and a plus or minus sign. The program trays, however, could be moved around and plugged in to send signals either to the entire machine or to specific units. These program trays were the electronic equivalent of the shafts that connected the various units of the Differential Analyzer.
The existence and function of ENIAC were made public in February of 1946 with print and other media communications. You can access the story that ran in The New York Times, complete with close-up photographs of Mauchly and Eckert, by clicking on the article at right.