Chapter 34 The engineering design of the Stretch computer 437

paths represented by transistor A or C. Which path is chosen by the current depends on the condition existing on base A. If point A is positive with respect to ground by 0.4 volts, that particular transistor is cut off, making the emitter of transistor C positive with respect to the base and, therefore, making C conducting. The current supplied by the current source (6 ma) will then flow through transistor C to the load f . Output f , then, is positive by 0.4 volts with respect to the -6 volt reference. This indicates at f the equivalent function impressed on A. At the same time, f is negative with respect to the -6 volt power supply by 0.4 volt, representing, therefore, the inverse of the function impressed on A. Conversely if A is negative with respect to the ground reference, transistor A is the conducting one, keeping emitter C negative with respect to its base. The current flows through transistor A, making f positive with respect to -6 and f negative with respect to -6. Again, the output of f reflects the function impressed on A, whereas f represents the inverse of the function.

If an additional transistor now is paralleled with A, it becomes obvious that only if both bases A and B are positive will output f be positive and f negative. If any or none of the bases A and B are positive, then f will be negative and f will be positive. In other words, an AND function is obtained on output f .

This principle, which is reflected in all the circuits, is essentially the principle of current switching or current steering.

Logical functions for the PNP circuits are, therefore, a + AND or - OR. Two outputs from each circuit block are available: the AND function and the inverse of the AND function.

A dual circuit exists for NPN transistors with input levels at -6 volts and output levels at ground. This circuit will give the + OR or - AND function.

A thorough investigation of the systems design showed that the circuits described so far are versatile enough to be used throughout the system. However, there are enough special cases (resulting from the many data buses and registers throughout the machine) that could use a distributor function or an overriding function. This caused the design of a circuit which permitted great savings in space and transistors by adding a third voltage level. Figure 12 shows the PNP version of the third-level circuit.