What should be done with unused TTL output?

Unused TTL inputs should never be left open. Depending on how their logic levels

will affect the operation you want from the device, unused inputs should always

be tied to either the +5v supply (logic 1) or to the circuit ground (logic 0). If an

input is left open ("floating"), then it can assume either logic level, and you literally

never know which state it's in.