WiredAND

Wired-AND is a digital logic technique that combines multiple outputs onto a single shared line using open-collector (or open-drain) outputs and a pull-up resistor. It enables an AND function without a dedicated logic gate, by letting any device pull the line low to indicate a logical 0 while the line is pulled high when no device is pulling it down.

In operation, each output transistor can sink current to ground. If any device asserts a low, the common line is driven low. If all devices release the line, the pull-up resistor pulls it high. Therefore the line represents the logical AND of the individual outputs when high corresponds to logic 1 and low to logic 0. This arrangement requires open-collector/open-drain outputs and a common supply, with proper matching of voltages and a suitable pull-up value.

Typical uses include bus-sharing scenarios, simple gating, and parallel signaling where multiple devices must agree before a signal is considered true. It is common in older TTL circuits and some microcontroller interfaces where open-collector outputs are available. The method benefits from simplicity and reduced part count and can support multiple inputs without risk of two push-pull drivers fighting each other.

Limitations include slower switching speeds due to the pull-up resistor, increased delay from line capacitance, and sensitivity to noise on long or high-capacitance lines. It also requires that all devices use open-collector or open-drain outputs and share a common supply; attempting to mix push-pull outputs can create contention. Wired-AND is related to wired-OR concepts and relies on passive pull-ups and negative-true logic in certain implementations.