Reference STATE OF THE ART Stan Augarten

ISBN 0-89919-195-9

Photo of
A Programmable Logic Chip
The PAL 16L8


Early logic chips contained a fixed number of logic gates and thus were said to be hardwired. At first, logic ICs were composed of only one or two gates, but the number rose each year, until hundreds and then thousands of gates were being placed on a single IC. Logic chips are the Boolean decision-makers of computers and other electronic equipment, but not all computers require precisely the same assortment of gates; a computer designed for scientific operations, for example, needs somewhat different internal circuitry from a word processor.

  Since it's wasteful to use more logic chips than a machine really needs, or to use chips with thousands of gates that serve no practical purpose in a particular machine, engineers developed programmable logic chips such as the one on the right. One of the first was the Micromosaic (p. 22); it contained some two thousand transistors that could be hooked up to create almost any pattern of up to a hundred and fifty gates by altering the chip's aluminum interconnections in accordance with the customer's requirements.

  Micromosaic and other customized chips greatly enhanced the versatility of ICs, but they were relatively expensive. Not only did the customer have to work closely with the chip-maker to design the right chip - and the two might be located thousands of miles apart - but the chip-maker had to modify the last stages of the fabrication process to produce the specific chip required. The in 1977, engineers John Birkner and H. T. Chua (the later had designed the 256-bit static RAM on p. 24) invented a logic IC that can be bought off the shelf, so to speak, and programmed by the customer in his own plant.

  Called PAL - an acronym for programmable array logic - the chip contains 2,048 tiny fuses. These fuses may be blown to create almost any configuration of up to two hundred and fifty AND, OR, and NOT gates. Blowing the fuses is a relatively simple procedure that opens some gates and closes others. Properly programmed, PAL can replace a dozen or more hardwired chips. Incidentally, a logic chip is not the same thing as a microprocessor. The latter is a self-contained calculating engine, whereas the former can perform only a limited series of operations and is often controlled by a microprocessor.

The PAL 16L8's logic gates are arrayed in a central grid that resembles a matrix of memory cells. The bright hues of this photo are the result of colored lights shone on the chip. Actual size: 0.133 x 0.138 inches. Photo of


©Copyright Stan Augarten
This book is provided for general reference. The National Museum of American History and the Smithsonian Institution make no claims as to the accuracy or completeness of this work.

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