Transistor Circuitry in the Lincoln TX-2
KENNETH H. OLSEN
Only two basic circuits are needed to perform most of the logical operations in the TX-2 computer: a saturated transistor inverter and a saturated emitter follower. To the logical designer who works with them, these circuits can be considered as simple switches that are either open or closed.
The schematic diagram of an emitter follower and the symbol used by the logical designers is shown in Figure 1. With a negative input, the output is "shorted" to the -3 V supply as through a switch. When several of these emitter followers are combined in parallel, as in Figure
Figure 1. Emitter follower.
2, any one of them will clamp the output to -3 V. We then have an OR circuit for negative signals and an AND circuit for positive signals. The transistor inverter is shown in Figure 3 with its logic symbol. Basic AND, OR circuits result from the connection of these simple switches in series or parallel (Figures 4 and 5). More complex networks like the TX-2 carry circuit use these elements arranged in series-parallel (Figure 6).
In Figure 3 the resistor R1 is chosen so that under the worst combinations of stated component and power supply variations, the drop
Figure 2. Parallel emitter follower.