96 IN THE BEGINNING

that needed to be stocked. A voltage adjusting (margining) system for identifying marginal circuits was another important feature of the TX-2 circuit design.

Today, computer engineers generally try to use a limited number of flip-flop types (or RAM types, etc.) because they have certain favorites whose characteristics they understand well and because the cost of bringing new parts into a company is very high. The old reasons - to simplify design, training, and stocking of spares - continue to apply as well. Even though keeping the number of different plug-in units (modules) to a minimum continues to have these advantages, this cannot be done as easily as it once was, principally because the increased functionality now available has customized modules to such a great degree. For example, in the case of an LSI- 11, the computer is a single module.

Modern designs do not use margining except in special cases where the refresh clock cycles of dynamic memories are altered to detect failures. However, special maintenance logic is often included in current designs. The idea of built-in maintenance features is in some ways similar to the old margining idea: in other ways it is a substantial deviation because additional parts are required, and the old de signers were extremely careful of the parts count. The emphasis on low component cost and parts count expressed in these chapters may seem odd to modern designers, but the gradual lessening of this concern (as discussed in Chapter 4) serves as an excellent example of the declining cost of electronic technology and of semiconductor technology in particular.

In summary, the modules chapters which follow form a starting point, both in time and in technology, for a study of how the views, concepts, and trends described in the first two chapters have applied in the development of DEC modules and computers.