Difference between revisions of "Negative logic"
From Computer History Wiki
m (better cat) |
m (fix link) |
||
| Line 3: | Line 3: | ||
The current usual meaning is [[logic]] in which the [[voltage]]s for 'true' and 'false' are reversed; for example, with the [[transistor-transistor logic|TTL]] [[logic family]], true would be [[ground]], and false would be +5V. The term '''active low''' is often used for this meaning. | The current usual meaning is [[logic]] in which the [[voltage]]s for 'true' and 'false' are reversed; for example, with the [[transistor-transistor logic|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 [[Digital Equipment Corporation|DEC]]'s [[System Module]]s, 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 [[input/output|I/O]] [[bus]]ses of some early DEC computers, such as early [[PDP-8]]'s. | + | An older meaning is the use of negative voltages. For example, this is seen in [[Digital Equipment Corporation|DEC]]'s [[System Module]]s, 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 [[input/output|I/O]] [[bus]]ses of some early DEC computers, such as early [[PDP-8 family|PDP-8]]'s. |
{{semi-stub}} | {{semi-stub}} | ||
[[Category: Hardware Basics]] | [[Category: Hardware Basics]] | ||
Latest revision as of 22:24, 22 November 2019
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.
