What logic state do TTL inputs develop when left open?

When TTL (Transistor-Transistor Logic) inputs are left open, they typically develop a high-logic state due to how TTL devices are designed. TTL circuits generally have input impedance characteristics that pull the input towards a high state when not actively driven.

This high state occurs because of the internal structure of TTL circuits that includes pull-up resistors or biasing that tends to pull the voltage at the input towards a logic high level (usually around +2 volts). Additionally, in TTL logic families, the internal transistor arrangement will not allow a floating state that could lead to undefined behavior, thereby defaulting to a high state.

Understanding this behavior is essential in circuit design and troubleshooting, as leaving inputs open can inadvertently lead to unintended high states, which can affect circuit functionality. In practical applications, it's common to use pull-down or pull-up resistors intentionally to manage the state of an input under conditions where it might otherwise be left open.