DIGITAL MODULES, THE BASIS FOR COMPUTERS 113

Figure 17. Diode gate.

The B-Series with blue handles was essentially the same as the 6000 Series of 10-MHz System Modules, except that it was repackaged on new 2.5- X 5-inch cards and used silicon transistors rather than germanium transistors. The new silicon transistors were a mixed blessing. While they had temperature sensitivity characteristics superior to those of the germanium transistors, and their voltage drop characteristics permitted the elimination of the bias resistor to + 10 volts, they did not saturate as well as the germanium transistors. Because they did not saturate well, the voltage between the collector and the emitter in the saturated state was not as low as it was with germanium transistors. This meant that the series arrangement of three inverters discussed in conjunction with the dotted lines in Figure 4 could not be used. Instead, only two of the silicon transistor in- be connected in series if the output was in tended to drive another inverter. The first computer to use the B-Series modules was the PDP 7, and the series was heavily used and extended by the first PDP-10 processor (KA10).

Figure 18. D-C-D gate.

Analog applications were the target market for the A-Series modules, which had amber handles. This series, still being manufactured today, includes analog multiplexers, operational amplifiers, sample and hold circuits, comparators, digital-to-analog converters, reference voltage supplies, analog-to-digital converters, and various accessory modules. The development rate of analog modules peaked in 1971 with 38 new types and declined to 5 new types in 1977.

While all of the preceding modules had been designed as user-arrangeable building blocks, the green handled G-Series was intended for