Negative logic
From Computer History Wiki
(Redirected from Active low)
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.
