852 Truth Tables

Logic circuits operate on the basis of Boolean logic and terms like, NOT, NOR, NAND etc., derive from Boolean algebra. This need not concern us here, but it is necessary to know that the input-output behaviour of logic devices is expressed in the form of a 'truth table'. The truth table for the NOR gate is given in Fig. 8.7.

Fig. 8.7 NOR gate symbol and truth table

In the NOR truth table, when the inputs A and B are both 0 the gate output, C, is 1. The other three input combinations each give an output C=1.

In computing and control systems, a system known as TTL (transistor to transistor logic) is often used. In TTL logic, 0 is a voltage between 0 and 0.8 V and logic 1 is a voltage between 2.0 and 5.0 V. A range of other commonly used logic gates and their truth tables is given in Fig. 8.8.

Type of logic gate

USA symbol

UK symbol

Truth table

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