threestate

Three-state, often written tri-state, is a concept in digital logic describing devices capable of producing three distinct electrical states: logic high, logic low, and a high-impedance state. The high-impedance state effectively isolates the device from a shared connection, such as a data or address bus, allowing multiple devices to be connected to the same line without causing short circuits when they are not actively driving the bus.

Tri-state devices are implemented with tri-state buffers or drivers that include an enable input. When the enable is active, the output drives the corresponding logic level (0 or 1). When the enable is inactive, the output enters the high-impedance state, making it appear as if the device is disconnected from the line. This arrangement enables multiple sources and sinks to share a single line by taking turns driving the bus.

Common applications include memory interfaces, peripheral I/O, and microprocessor buses, where several devices must access a single data path without interfering with one another. To maintain a defined idle state on unused lines, external pull-up or pull-down resistors or proper termination may be used.

Implementation details and considerations: tri-state logic is typically realized with complementary transistor networks in CMOS technology, controlled by an enable signal that disables both the pull-up and pull-down paths. Designers must ensure that only one device drives the bus at any given time, manage timing and fan-out, and mitigate potential data contention or signal integrity issues. Variants include tri-state buffers, bus transceivers, and multiplexed bus controllers.