112 IN THE BEGINNING
By 1964, because of the decreasing cost of semiconductors during the early 1960s, the cost of System Module mounting hardware and of
Figure 16. Single and double Flip Chip modules used in PDP-7 and PDP-8.
wiring had become a significant portion of the total system cost. In response to this trend, a new type of module was developed which was a 2.5- X 5-inch printed circuit card with a color- coded plastic handle (Figure 16). The printed circuit card provided its own mechanical support - there was no metal frame around it as there had been in the System Module design. The new modules, called Flip Chip modules, plugged into 144-pin connector blocks that could support eight such modules, providing 18 pins per module. While the improvements in the cost of module mounting hardware realized with the new modules were important, the major advantage of the new Flip Chip modules was that automatic Gardner-Denver Wire-wrap equipment could be used to wire the module mounting blocks.
The first series of the new modules was designated the R-Series and was identified by using red handles. The R-Series circuits were a reaction to the rather complicated set of rules developed for using the previous products. The goal was to make these modules easy to use and inexpensive. Integrated circuits were not used because they were more expensive than discrete components, and the computer industry had not yet decided on the type of integrated circuit to use. The building block for R-Series logic was the diode gate, an example of which is shown in Figure 17. The other basic circuit was the diode-capacitor-diode (D-C-D) circuit shown in Figure 18. The diode-capacitor-diode gate was used to standardize inputs to active devices such as flip-flops and to produce the logic delay necessary to sense and change flip-flops at the same time.
A second series of the new modules was developed for the first PDP-8s. This series was called the S-Series, although it also had red handles. The S-Series modules used the same circuits as their R-Series counterparts, but with variations in the values of the load resistors and diode-capacitor-diode gate storage capacitors to obtain greater speed.