Ttl

In electronics, TTL refers to a way of doing logic in circuits

historically, this Transistor-Transistor Logic (transistors perform both the logic as well as any amplfication) replaced earlier styles, mostly resistor–transistor logic and diode–transistor logic

practically often just means "logic at 5V"

which now is often in contrast with newer, lower-voltage (often 3.3V) logic levels

As a file extension, .ttl may point to turtle format of storing triplets