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Text 18 RELAY COMPUTERS

Even in the 40s the primary speed limit on these rudimentary computers was mechanical, so developers looked to other technologies to build their computers. Bell Telephone Laboratories began work on relay-based computers in 1938. A relay is an electrically controlled switch-one source of electricity activates an electromagnet which operates a switch which, in turn, alters the electrical flow in another circuit. Relays are a hybrid technology, electro-mechanical. Their mechanical side performs physical work while their electrical nature makes them very flexible. One relay can control others almost unlimited in number and distance. The gears and fevers of

purely mechanical calculators are limited in reach in both regards.

The choice of relay technology was a natural one for the telephone company. After all, the telephone switching systems of the time made extensive use of relays-rooms and rooms fitted with them.

Electronic Computers

Early in the development of the computer, designers recognized the speed advantages of an all electronic machine. After all, electronic signals could switch thousands or millions of times faster than mechanical cams or electrical relays.

The first successful electronic machine was secretly developed as part of the British cryptoanalysis program at Bletchley Park during World War II. There, T. H. Flowers created an electronic machine known as Colossus for comparing cipher texts. Colossus, first tested in December 1943, pioneered the concept of electronic clocked logic (with a clock speed of 0.005 MHz) and used 1,500 vacuum tubes.

Although Colossus was a programmable machine, neither it nor the succeeding generations of cryptographic machines developed at Bletchley Park were designed to handle decimal multiplication. Moreover, the development of Colossus and its kin was kept secret until long after the war, so it did not in itself contribute to the development of the computer. In fact, many details of the Bletchley Park operation are still secret forty years later.

The seminal machine in the history of the electronic computer is generally regarded as ENIAC, the Electronic Numerical Integrator and Computer, developed at the Moore School of the University of Pennsylvania in Philadelphia by a team led by John Mauchly and J. Presper Eckert. Proposed in 1943, it was officially inaugurated in February 1946. The most complex vacuum tube machine ever built, ENIAC occupied a 30 by 50 foot room (at 1,500 square feet, that's the size of a small house), weighed 30 tons, and required about 200 kilowatts of electricity. It used 18,000 vacuum tubes and was based on a clocked logic design. When operating at its design clock speed of 0.1 MHz, ENIAC required a mere 200 microseconds for addition, and 2.6 milliseconds for multiplication. At about 5,000 arithmetic operations per second, it was approximately 1,000 times faster than the Harvard Mark I.

The design goal of ENIAC was to calculate ballistic trajectories, and the machine succeeded well. It was able to compute the path of a 16-inch artillery shell in less than real time-that is, it could predict about where a shell would fall after it was fired but before it hit.

The next step in the development of the computer and PC was EDVAC, the Electronic Discrete Variable Automatic Computer. Unlike the decimal-based ENIAC, EDVAC was designed as a binary computer. Information to EDVAC was encoded in its most essential form- the presence or absence of a code symbol-which could be represented by a voltage. This binary basis is the essence of today's digital logic, upon which nearly aH current computers are based.

With UNIVAC, the basic operating principles of the computer were in place. Further

developments have come in the refinement of the technology used to make computer circuits. Switching from tubes to transistors increased reliability and allowed designs to become both more complex (mainframes) and smaller (minicomputers). Memory shifted from mercury delay lines (which briefly stored data as ultrasonic pulses propagating through tubes of liquid mercury) and cathode ray tubes to magnetic core and finally solid-state transistors. Integrated Circuits continued this trend and made possible microprocessors and RAM chips, which, in turn, led to the circuits that formed the basis of the first personal computers.