Positive and Negative Logic in digital systems

The binary variables can have either of the two states, i.e. the logic ‘0’ state or the logic ‘1’ state. These logic states in digital systems such as computers, for instance, are represented by two different voltage levels or two different current levels. Positive and negative logic systems come into picture here.

Table Of Contents

Positive and Negative Logic – definitions
Positive and Negative Logic – examples
positive OR is a negative AND

Positive and Negative Logic – definitions

If the more positive of the two voltage or current levels represents a logic ‘1’ and the less positive of the two levels represents a logic ‘0’, then the logic system is referred to as a positive logic system.

If the more positive of the two voltage or current levels represents a logic ‘0’ and the less positive of the two levels represents a logic ‘1’, then the logic system is referred to as a negative logic system.

Positive and Negative Logic – examples

The following examples further illustrate this concept of Positive and Negative Logic.

If the two voltage levels are 0 V and +5 V, then in the positive logic system the 0 V represents a logic ‘0’ and the +5 V represents a logic ‘1’.

In the negative logic system, 0 V represents a logic ‘1’ and +5 V represents a logic ‘0’.

If the two voltage levels are 0 V and −5 V, then in the positive logic system the 0 V represents a logic ‘1’ and the −5 V represents a logic ‘0’. In the negative logic system, 0 V represents a logic ‘0’ and −5 V represents a logic ‘1’.

positive OR is a negative AND

It is interesting to note, that a positive OR is a negative AND. That is, OR gate hardware in the positive logic system behaves like an AND gate in the negative logic system. The reverse is also true. Similarly, a positive NOR is a negative NAND, and vice versa.