On 1/29/12 1:50 PM, threeneurons wrote:
Believe it or not, digital logic predates ICs. The British Colosus to ENIAC, UNIVAC, and all the computers built to the late 50s used vacuum tubes.
But with discrete components, you have a lot of options for your logic. Yes, you can build binary counters, and decode the binary or BCD results, or you can use ring counters which needs no further decoding.
ENIAC was a very interesting computer. It was essentially an adding machine, built with vacuum tube decimal counting wheels. Each decade was a ring counter with a "count up" command. Numbers traveled through the machine as series of pulses, one wire per decade.
The circuitry was very weird by modern standards also. The designer of the electronics was a radio designer, and he used a bizarre DC coupled logic ladder method that required many power supplies. Essentially, each plate was coupled directly to the grid of the tube in the next logical stage, so the voltage started at about -300V and worked its way up to +300V through the logic ladder.
The weird electrical design was one of many reasons that ENIAC was completed a year after its need, WWII, had ended. The crash design program had resulted in a machine that took forever to actually build.
In the early Fifties, IBM figured out how to make a tube logic family that used diode-clamped direct coupling which ran at a standard set of voltages, much like the later TTL logic family. This allowed them to design a new computer in about one year.
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