What is TTL logic?

TTL Logic Circuits - "Transistor Transistor Logic" - are made using BJTs. They operate from +5 Volts. These are available from simple AND, OR, NAND Gates to complex circuits.

The voltage levels are

INPUT OUTPUT

Low (0) < 0.8V <0.4V

High (1) >2.0V >2.4V

Levels in between are undefined. The outputs are better than the inputs by 0.4V thus offering a noise margin. These are seldom used these days as other logic families have been introduced offering more advantages like, speed, lower power, wider / lower operating voltage, higher fanouts.