Wired logic connection

A wired logic connection is a logic gate that implements boolean algebra (logic) using only passive components such as diodes and resistors. Limitations include the lack of amplification to provide level restoration and its constant ohmic heating for most logic (particularly more than CMOS) which indirectly limits density of components and speed.

Wired logic works by exploiting the high impedance of open collector outputs (and its variants: open emitter, open drain, or open source) by just adding a pull-up or pull-down resistor to a voltage source, or can be applied to push–pull outputs by using diode logic (with the disadvantage of incurring a diode drop voltage loss).

Active-high wired OR connection

See also: Diode logic § Active-high OR logic gate

Open-emitter buffers connected as wired OR using open emitter or similar inputs (which can be identified by the ⎏ symbol in schematics) connected to a shared output with a pull-down resistor. This gate can also be easily extended with more inputs.

The wired OR connection electrically performs the Boolean logic operation of an OR gate.

When all inputs are LOW, they all present high impedance, and the pull-down resistor pulls the output voltage LOW, but if any input is HIGH, they pull the output HIGH:

Inputs		Output
A	B	A OR B
LOW	LOW	LOW
LOW	HIGH	HIGH
HIGH	LOW	HIGH
HIGH	HIGH	HIGH

When driving a load, the LOW output is raised by the pull-down's voltage drop, though the HIGH output is almost the supply voltage (5V). But if diode logic is used, each input requires a diode, and the HIGH output voltage will additionally be lowered by the diode's forward voltage.

Active-high wired AND connection

See also: Diode logic § Active-high AND logic gate

Diode circuit implementing AND in active-high logic.

The wired AND connection is a form of AND gate.

When all inputs are HIGH, they all present high impedance, and the pull-up resistor pulls output voltage HIGH, but if any input is LOW, they pull the output LOW:

Inputs		Output
A	B	A AND B
LOW	LOW	LOW
LOW	HIGH	LOW
HIGH	LOW	LOW
HIGH	HIGH	HIGH

When driving a load, the HIGH output is reduced by the pull-up's voltage drop. If diode logic is used, each input requires a diode, and the LOW output voltage will additionally be raised by the diode's forward voltage. Care should be taken to ensure the output still lies within valid voltage levels.

Reversing active level

An active-high wired AND can be treated as active-low wired OR (and an active-high wired OR can be treated as active-low wired AND), by using active-low logic (or negative logic) and applying De Morgan's laws.

Compatibility of wired AND OR using diodes

Main article: Diode logic

Wired AND in diode logic

Wired OR in diode logic

Diode logic uses a diode for each input in addition to a shared pull-up resistor (for wired AND) or a pull-down resistor (for wired OR). However, each stage of diode logic reduces output voltage levels. So without amplification, the output voltage may not be compatible with the primary logic family. Diode logic alone can't create a NOT gate.

References

• Digital Techniques, Heathkit Educational Systems,1990
• Fundamental Physics, K.L Gomber and K.L Gogia, Pradeep Publications, 2005

External links

• Configuring NI 6xxx Devices for Wired-OR (Open Collector) Output Mode In LabVIEW – An Introduction to Wired-OR Outputs and Open-Collector Circuits