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438 Part 5 The PMS level

Section 2 Computers with one central processor and multiple input/output processors

If transistor X were eliminated, then transistors A and B in conjunction with the reference transistor C would work normally as a current switching circuit, in this case a + AND circuit. If transistor X is added with the stipulation that the down level of X is more negative than the lowest possible level of A or B, it becomes apparent that when X is negative, the current will flow through that branch of the circuit in preference to branch f or f , regardless of inputs A and B. Therefore, the output of f and f will be negative, provided input X is negative. Output III is the inverse of input X. If, however, X is positive, then the status of A and B will determine the function f and f implicitly. This demonstrates the overriding function of input X.

Similarly, the NPN version (not shown) results in the OR function of f if input X is negative and in a positive output at f and f , regardless of status A and B, if X is positive. Again minimum and maximum signal swings are shown in Fig. 12.

The speed of the circuits described so far depends on the number of inputs and the number of circuits driven from each load. The response of the circuit is anywhere between 12 and 25 mm sec per logical step with 18 to 20 mm sec average. The number of inputs allowable per circuit is eight. The number of driven circuits is three. Additional circuits are needed to drive more than three bases and where current switching circuits communicate over long lines, termination networks must be added to avoid reflections.

To improve the performance of the computer in certain critical places, emitter-follower logic is used as shown in Fig. 13. These circuits, having a gain less than one, after a number of stages require the use of current switching circuits as level setters and gain devices. Both AND and OR circuits are available for both a ground-level and a - 6-level input. Change from a - 6-level circuit to a ground-level circuit is obtained by applying the appropriate power supply levels. Due to the variations in inputs and driven loads, the circuits must be designed so that the load can vary over a wide range. This resulted in instability which had to be offset by the feedback capacitor C shown in the circuit.

All functions needed in the computer can be implemented by the use of the aforementioned circuits, including flip-flop operation, which is obtained by tying a PNP current switch block and an NPN current switch block together with proper feedback.

Packaging

The circuits described in the last paragraph are packaged in two ways:

A circuit package using the smaller of the two printed circuit boards shown in Fig. 14, called a single card, contains AND or OR circuits. It should be mentioned that the printed wiring is one-sided and that besides the components and transistors, a rail is added which permits the shorting or addition of certain loads depending on the use of the circuits. This rail then has the effect of reducing the different types of circuit boards in the machine. Twenty-four different boards are used and of these, two types reflect approximately 70% of the total single card population.

Due to the large number of registers, adders, and shifters used in the computer, it seems reasonable that functional packages could be employed economically, because of wide usage. This results in the high-density package also shown in Fig. 14, called

Fig. 15. The back panel.

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