TTL

Transistor-to-Transistor Logic (TTL) used to be one of the most common I/O standards. TTL operates from a +5V or 3.3V power supply and, at one time, had higher transmit speeds compared to CMOS (up to 100MHz). It was also more popular because power consumption didn’t change as drastically with higher output frequencies. TTL outputs can also be dealt with using methods described for CMOS signals. During the 1980s, CMOS devices became more popular, particularly for large scale integration because of their low (zero) quiescent current, good noise immunity, improved rise/fall times, and lower cost of manufacture. CMOS has displaced TTL as the preferred choice for low frequency clocking ICs.

The primary benefits of CMOS and TTL are low power consumption, higher output swing, and relatively low cost implementation in silicon. However, differential signals are used for higher frequencies.