In digital electronics, what is meant by tri state (three state) logic on an output or bus line?

Introduction / Context:
Tri state logic, sometimes written three state logic, is a fundamental concept in digital electronics, especially when multiple devices share a common bus. Standard digital outputs normally have two valid logic levels: logic 0 and logic 1. Tri state outputs add a third possible state, which is neither logical zero nor logical one. Understanding this concept is important when designing microprocessor buses, memory interfaces and other systems where many chips must safely share the same physical lines.

Given Data / Assumptions:

We are dealing with digital outputs that drive signals on a bus or line.
Normal logic levels are represented by voltage ranges corresponding to logic 0 and logic 1.
Tri state logic introduces an additional state beyond these two normal levels.
The third state is sometimes called high impedance or high Z, indicating a very high effective output resistance.
We are not describing oscillating outputs, multi voltage supplies or always active multi pin circuits.

Concept / Approach:
In tri state logic, an output driver can be enabled or disabled. When enabled, it behaves like a normal digital driver and forces the line to logic 0 or logic 1 depending on the input and circuitry. When disabled, the driver effectively disconnects itself from the line by going into a high impedance state. In this state, it neither sources nor sinks significant current, and does not interfere with other devices that may be driving the same line. This allows multiple devices to share a bus safely, as long as only one device drives the bus at a time and all others are in the high impedance state.

Step-by-Step Solution:
Step 1: Recall that digital signals require clear logic 0 and logic 1 levels for correct interpretation by receiving circuits. Step 2: Recognise that when more than one device is connected to the same line, there is a danger that one device might drive the line high while another drives it low, causing contention and potentially damaging current. Step 3: Understand that tri state outputs solve this by allowing devices that are not currently transmitting to place their outputs into a high impedance state, which is effectively disconnected from the bus. Step 4: Note that the high impedance state is not a third logic level to be decoded by receivers; instead, it is an electrically disconnected state that leaves the line free to be driven by another device or to be held by pull up or pull down resistors. Step 5: Therefore, tri state logic means that an output can be in three states: logic 0, logic 1 or high impedance.

Verification / Alternative check:
Look at common microcontroller or memory data sheets. You will often see timing diagrams describing when bus lines are driven and when they are in high Z. Control signals such as OE (output enable) or EN (enable) explicitly control whether the device drives its outputs or leaves them in tri state. Logic analyzers may indicate high impedance as Z in captured waveforms. These practical examples match the definition given in the correct option.

Why Other Options Are Wrong:
An output that cycles between 0 and 1 without a stable state describes an oscillating or unstable signal, not tri state logic.
Using three supply voltages is unrelated to the concept of three state outputs and is not how tri state logic is defined.
Driving exactly three output pins at the same time does not describe a logic state; it is simply a wiring choice.

Common Pitfalls:
Beginners sometimes mistakenly think that the high impedance state is a third logic level that receivers can directly interpret, but in reality it simply represents a disconnected condition. Another common error is to forget to manage which device is driving the bus, leading to two drivers active at the same time and causing contention. Correct design uses control signals and bus protocols to ensure only one active driver and all others in high Z when sharing lines.

Final Answer:
Tri state logic describes an output that can be in three states: logic 0, logic 1 or a high impedance disconnected state, which is essential for safe bus sharing.