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 OR" logic connection using a pull-down resistor after two open-emitter buffers (the ⎏ symbol indicates an open emitter or similar output). This OR 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.
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.
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.
Applications
Simplify computer bus connection
Wired-OR signaling is common for multipoint bus standards, e.g. Controller Area Network (CAN), Small Computer Systems Interface (SCSI), IEEE-488 (GPIB), and IEEE-896 (BTL). It provides collision detection and OR gating of all outputs on signal line(s). Wired-logic is often used by multipoint busses during bus arbitration and sometimes during data transfer. It simplifies sharing a bit of information among separated nodes without the cost of a sophisticated protocol.
