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As the various module circuit technologies developed, more logic functionality fit in a given space, and the space provided on individual logic modules was increased. The modules used in the PDP-8/I, PDP-8/L, PDP-10 (KI10 processor), and PDP-15 were single (2.5 X 5- inch) and double (5 X 5-inch) general purpose modules, and these machines had relatively low packing densities because most interconnections were carried out on the wired backplane. The PDP-8/E (and, to a lesser extent, the PDP-l 1/20) used 8.5 X 10.4-inch "extended quad" modules which were functionally specialized and eliminated many of the backplane connections required in previous designs. By 1973, the "hex" module (8.5 X 15.6 inches) was widely used, principally in the PDP-1 1 family. By 1978 two DEC computers, the VAX 11/780 (1977) and the DECSYSTEM 2020 (1978), were using 12 X 15.6-inch "super hex" modules to-further reduce interconnection cost by placing more logic on a single module.

An evolution in circuits has continued as the technology has changed. As integrated circuits have become more functional by the reduction of the size of their active elements, each new computer introduced is smaller, faster, and less costly than its predecessor. While only DEC examples have been mentioned here, the trend to ward smaller, faster, and less costly computers has been consistent for all computer manufacturers.

The chart in Figure 22 shows the number of module types introduced each year from 1957 to 1977.


We gratefully acknowledge the review assistance offered by Allan Kent, Tom Stockebrand, Phil Tays, and Don White.


. PDP-8.

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