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The following diode logic gates work in both active-high or active-low logic, however the logical function they implement is different depending on what voltage level is considered active. Switching between active-high and active-low is commonly used to achieve a more efficient logic design.
Diode–transistor logic (DTL) is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating functions AND and OR are performed by diode logic , while logical inversion (NOT) and amplification (providing signal restoration) is performed by a transistor (in contrast with ...
A family of diode logic and diode–transistor logic integrated circuits was developed by Texas Instruments for the D-37C Minuteman II Guidance Computer in 1962, but these devices were not available to the public. A variant of DTL called "high-threshold logic" incorporated Zener diodes to create a large offset between logic 1 and logic 0 ...
These open outputs configurations are often used for digital applications when the transistor acts as a switch, to allow for logic-level conversion, wired-logic connections, and line sharing. External pull-up/down resistors are typically required to set the output during the Hi-Z state to a specific voltage.
A level shifter connects one digital circuit that uses one logic level to another digital circuit that uses another logic level. Often two level shifters are used, one at each system: A line driver converts from internal logic levels to standard interface line levels; a line receiver converts from interface levels to internal voltage levels.
Diode–transistor logic improved the fan-out up to about 7, and reduced the power. Some DTL designs used two power-supplies with alternating layers of NPN and PNP transistors to increase the fan-out. Transistor–transistor logic (TTL) was a great improvement over these. In early devices, fan-out improved to 10, and later variations reliably ...
In most types of logic design, termed static logic, there is always some mechanism to drive the output either high or low. In many of the popular logic styles, such as TTL and traditional CMOS, this principle can be rephrased as a statement that there is always a low-impedance DC path between the output and either the supply voltage or the ground.
The same diode circuits are used, except a switch is placed in series with each diode. To read whether a switch is open or closed, the microcontroller configures one pin as an input with an internal pull-up resistor. The other pin is configured as an output and set to the low logic level.