DIGITAL MODULES, THE BASIS FOR COMPUTERS 105

Figure 4. Symbolic drawing of an inverter.

source, were it not for the clamp diode which limits the voltage of the collector to -3 volts.

To facilitate maintenance, the + 10-volt bias supply shown in Figure 3 was adjustable for margin checking, a feature which had been used in the TX-2 and which is discussed in Chapter 4.

To simplify the logic drawings, a symbolic drawing like that in Figure 4 was customarily used to represent the inverter circuit. Note that neither Figure 3 nor Figure 4 shows the emitter directly connected to ground or the collector directly connected to the negative supply. Rather, a dotted line is used on the drawings to indicate that Laboratory Modules and System Modules often used a series connection of up to three inverter gates between the negative supply and ground to accomplish various logic functions. Parallel and series-parallel arrangements were also used, as shown in the sample circuits in Figure 5.

The Digital Laboratory Modules and the Digital System Modules used a dual polarity logic system employing both levels and pulses. The logic voltage levels were -3 volts and ground. Correspondence between the logic state, ONE or ZERO, and the voltage levels of -3 and ground were indicated at each point in the logic diagram by a diamond. The diamond

Figure 5. Sample circuits using series and parallel arrangements of inverters.

defined the necessary voltage level for the action desired. A solid diamond denoted that a -3-volt level was an assertion, and a hollow diamond indicated that a ground level was an assertion. This convention gave two signal names to one physical signal: if a given asserted signal A was passed through an inverter, four signals resulted, as shown in Figure 6.

A logic function lower in cost yet equivalent to both the series and parallel inverter arrangements used diodes added to the circuit of Figure 3 to form AND or OR gates, as shown in Figures 7 and 8.

Except for very small amounts of delay, the inputs and outputs of these circuits changed simultaneously; thus, no information was stored. The storage of information was accomplished by bistable devices called "flip-flops" whose state was controlled by the application of pulses. Before discussing the construction of flip- flops, it is therefore necessary to briefly describe pulses, which were an important type of logic signal.

A pulse, as the name implies, was a very well controlled, short event in which a logic signal was asserted. Pulses were used for computer clocks and for carrying out the register transfer operations between the registers. Pulses were