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other forms are appearing. The calculator watch, the calculator pencil, the calculator alarm clock, and the calculator checkbook have all been advertised.

Between the two extremes of super computers and calculators, packaging has also been important in minicomputers and large computers. In particular, packaging seems to be the dominant reason for the success of the PDP-8 and the minicomputer phenomenon, although marketing, the coining of the name, and the ease of manufacture (also part of packaging) are alternative explanations. The principal packaging advantage of the PDP-8 over predecessor ma chines was the half-cabinet mounting which permitted it to be placed on a laboratory bench or built into other equipment, both locations being important to major market areas.

The Packaging Design Problem

The importance of packaging is equalled only by its complexity. The complexity stems from the range of engineering disciplines involved. Packaging is the complete design activity of interconnecting a set of components via a mechanical structure in order to carry out a given function. To package a large structure such as a computer, the problem is further broken into a series of levels, each with components that carry out a given function. Figure 1 shows the hierarchy of levels that have evolved in the last twenty years for the DEC computers. There are eight levels which describe the component hierarchy resulting in a computer system.

For each packaging level there is a set of interrelated design activities, as shown in Figure 2. The activities are almost independent of the level at which they are carried out, and some design activities are carried out across several levels.

Figure 1. Eight-level packaging hierarchy for second to fourth generation computer systems.

While the initial design activities indicated in Figure 2 are each aimed at solving a particular problem, the solving of one problem in computer engineering usually creates other problems as side effects. For example, the integrated circuits and other equipment that do information processing require power to operate. Power creates a safety hazard and is provided by power supplies that operate at less than 100 per cent efficiency. These side effects create a need for designing insulators and providing methods of carrying the heat away from the power sup ply and the components being powered. In this way, cooling problems are created. Cooling can be accomplished by conducting heat to an out side surface so that it may be carried away by the air in a room. Alternatively, cooling can be done by convection: a cabinet fan draws air across the components to be cooled and then carries the heated air out of the package into the

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