DIGITAL MODULES, THE BASIS FOR COMPUTERS 115

Figure 19. K-Series circuit.

Unlike other DEC modules, the K-Series modules were not directly useful for constructing computers or computer data processing subsystems due to their low speed and high cost. They did play an important part in bringing digital logic into industrial applications, and the noise protection techniques developed for these modules were useful in the design of the PDP-14 Industrial Controller (Chapter 7).

By 1967 the electronics world had settled on transistor-transistor logic (TTL) and the dual in-line package (DIP) as the technology and package of choice for integrated circuits. In addition, the cost for logic functions implemented in TTL integrated circuits had dropped below that of discrete circuit implementations. With much more logic fitting into the same printed circuit board area, a single Flip Chip card could now accommodate much more complicated functions. However, there were not enough connector pins available to get the necessary signals on and off the card. The answer to the problem was to keep the cards the same size, but to have etch and associated contacts on both sides of the printed circuit board. This increased the number of contacts from 18 to 36, and a new series with magenta handles (the M-Series) was born. Subsequently, some G-Series and W-Series modules were also designed with integrated circuits and double-sided boards.

Figure 20. Basic TTL NAND gate circuit.

The advent of transistor-transistor logic brought the first power supply and signal level change in DEC's history. The -15-volt and +10-volt supplies were no longer required. Only a single +5-volt supply was needed to sup ply the logic signals which were now 0 and +3 volts. The packaging was kept consistent, how ever, as the old single-sided modules could be plugged into the new connector blocks. Careful attention to pinning arrangements allowed half of the circuits of a double-sided module to be used in a single-sided block.

The basic TTL circuit is the NAND gate shown in Figure 20. Since the change to TTL logic brought a change in logic symbols, a sample of the new symbology is also shown in Figure 20.

The input of the TTL gate is a multiple emitter transistor. If either input is at or near ground (0 to 0.8 volts), transistor Q1 becomes saturated, bringing the base voltage of transistor Q2 low, turning off transistor Q3 while turning on transistor Q4, and making the output high (+2.4 to +3.6 volts). If both inputs are high (above 2 volts), Q2 has base current sup plied to it through the collector diode of Q1, turning Q2 on. This in turn provides base cur rent to Q3, saturating it and cutting off Q4, making the output low (0 to 0.4 volts).

Like the transistor inverter circuits discussed in conjunction with System Modules, TTL NAND gates can be cross-connected to form flip-flops.