In digital electronics three-state, tri-state, or 3-state logic allows output ports to have a value of 0, 1, or Hi-Z. Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical A logic gate performs a logical operation on one or more logic inputs and produces a single logic output A Hi-Z output puts the pin in a high impedance state, effectively removing the pin from its influence on the circuit. In Electronics, high impedance (also known as hi-Z, tri-stated, or floating) is the state of an output terminal which is not currently driven The intent of this state is often to allow multiple circuits to share the same output line or bus, or allow a device to monitor signals without affecting the signal (as used in Analog to Digital Converters). In Computer architecture, a bus is a subsystem that transfers data between computer components inside a Computer or between computers Tri-state is a registered trademark of National Semiconductor but is often used to describe devices made by any manufacturer.

Three-state outputs are implemented in various families of digital integrated circuits such as the 7400 series of TTL gates, and often in the data and address bus lines of microprocessors. Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside The 7400 series of Transistor-transistor logic Integrated circuits are historically important as the first widespread family of TTL integrated circuit Transistor–transistor logic ( TTL) is a class of Digital circuits built from Bipolar junction transistors (BJT and Resistors It is called A microprocessor incorporates most or all of the functions of a Central processing unit (CPU on a single Integrated Three-state outputs may be found on individual logic gates, or in multiples in one integrated circuit package as a buffer for connection to a bus.

On some bus drivers or port expander ICs, a "high" on a single input pin on the package called /OE (output enable) puts all the outputs in a Z state; a "low" on that pin makes all the outputs active, driving the bus to either 1 or 0.

This form of gate should not be confused with 3-value logic (ternary logic). A ternary, three-valued or trivalent logic (sometimes abbreviated 3VL) is a term to describe any of several Multi-valued logic systems in which

A tristate buffer can be thought of as a switch. If B is on, the switch is closed. If B is off, the switch is open.

Uses of three-state logic

The whole concept of the third state (Hi-Z) is to effectively remove the device's influence from the rest of the circuit. If more than one device is electrically connected, putting an output into the Hi-Z state is often used to prevent short circuits (when one device drives high (logical 1) against another device driving low (logical 0)).

Three-state buffers can also be used to implement efficient multiplexers, especially those with large numbers of inputs. In Electronics, a multiplexer or mux ( occasionally the term muldex is also found for a combination multiplexer-demultiplexer

Three-state logic can reduce the number of wires needed to drive a set of LEDs (tristate multiplexing or Charlieplexing). Charlieplexing is an electronics technique developed by Charlie Allen (hence the name at Maxim in which relatively few I/O pins on a Microcontroller are used

Use of Pull-Ups / Pull-Downs

When outputs are tri-stated (in the Hi-Z state, also known as Floating) their influence on the rest of the circuit is removed. Circuit designers will often use pull-up or pull-down resistors (usually with in the range of 1 - 100k ohms) to influence the circuit when the output is tri-stated. For example, the I2C bus protocol (a bi-directional communication bus protocol often used between devices) specifies the use of pull-up resistors on the two communication lines. I²C ( Inter-Integrated Circuit) is a multi-master serial Computer bus invented by Philips that is used to attach low-speed peripherals When devices are inactive, they "release" the communication lines and tri-state their outputs, thus removing their influence on the circuit. When all the devices on the bus have "released" the communication lines, the only influence on the circuit is the pull-up resistors, which pull the lines high. When a device wants to communicate, it comes out of the Hi-Z state and drives the line low. Devices communicating using this protocol either let the line float high, or drive it low -- thus preventing any short circuit situation where one device drives a line high and another low.

Alternatives to 3-state bus

A 3-state bus is typically used between chips on a single PCB, or sometimes between PCBs plugged into a common backplane. A backplane (or "backplane system" is a circuit board (usually a Printed circuit board) that connects several connectors in parallel to each other so

An open collector bus is often used between PCBs plugged into a common backplane, or connected over longer cables. Open collector is a type of output on many integrated circuits (IC.

Usage of three-state logic is not recommended for on-chip connections but rather for inter-chip connections. Basic multiplexers can be used on chip since the number of components and wires is not limited to the extent as in PCB (printed circuit board). ^[1]

See also

• Ternary logic
• Buffer

Sources

1. ^ "On-Chip Buses/Networks for SoC" "On-Chip Buses [have] No use of tri-state signals [because] Tri-state bus is difficult for static timing analysis"

What's a Tri-state Buffer?

Principle of Tristate Multiplexing