TTL

TTL stands for Transistor Transistor Level, and is an older specification for the voltage levels used to distinguish digital signals.

Voltage Levels in TTL are from 0 to 5 volts.

Other important TTL Voltages:
 * VIH: 2 Volts
 * VOH: 3 Volts
 * VIL: 0.8 Volts
 * VOL: 0.35 Volts
 * VCC: 5 Volts