Negative logic

Negative logic has two meanings.

The current usual meaning is logic in which the voltages for 'true' and 'false' are reversed; for example, with the TTL logic family, true would be ground, and false would be +5V. The term active low is often used for this meaning.

An older meaning is the use of negative voltages. For example, this is seen in DEC's System Modules, which use -15V and +10V supplies, and use -3V for the standard 'true' output level, and ground for 'false'. These levels are found in the I/O busses of some early DEC computers, such as early PDP-8's.