Standard Logic ICs: the Foundation of Digital Circuitry

There are roughly 600 types of standard logic ICs, from basic chips to highly functional arithmetic-and-logic units. There are two different types of implementation: TTL and CMOS.

◊ TTS (transistor-transistor logic) ICs: The main circuitry is built with bipolar transistors. These chips run on 5-V power.

◊ CMOS (complementary metal oxide semiconductor) ICs: The main circuitry is built from pairs of p-type and n-type metal oxide semiconductor field-effect transistors (MOSFETs). Voltages used to drive these chips extend over a wide range.

If a design calls for signals to be passed among digital ICs, then the designer must know the logical conditions that will produce an H or L, and the voltage ranges that represent these conditions. Voltage ranges corresponding to logical conditions are called logic levels. Communicating ICs must use the same logic levels; use of different levels will disable communication and may even cause IC breakage.
By convention, TTL ICs use the following levels.
◊ Input signals: 0 V to 0.8 V is L; 2.0 V and up is H.
◊ Output signals: 0 V to 0.4 V is L; 2.4 V and up is H.

TTL IC makers must design their ICs to meet the TTL interface standards indicated above. Since these values are now standard, logic designers working with TTL IC communication need not concern themselves with logic levels.
The case is different for CMOS ICs, where there are many different series with different logic levels, and where logic levels may also differ according to the supplied voltage. This means that designers must take care to use consistent logic levels when connecting different CMOS ICs.