Logic Families

The types of logic devices are classified in "families", of which the most important are TTL and CMOS. The main families are:

• TTL (Transistor-Transistor Logic), made of bipolar transistors.
• CMOS (Complementary Metal Oxide Semiconductor) made from MOSFETs
• ECL (Emitter Coupled Logic) for extremely high speeds
• NMOS, PMOS for VLSI large scale integrated circuits.

TTL Logic Family

The transistor-transistor-logic (TTL) family was developed in the use of transistor switches for logical operations and defines the binary values as

TTL is the largest family of digital ICs, but the CMOS family is growing rapidly. They are inexpensive, but draw a lot of power and must be supplied with +5 volts. Individual gates may draw 3 to 4 mA.

The low power Schottky versions of TTL chips draw only 20% of the power, but are more expensive. Part numbers for these chips have LS in the middle of them.

CMOS Logic Family

The complementary metal oxide semiconductor family (CMOS) has equivalents to most of the TTL chips. CMOS chips are much lower in power requirements (drawing about 1 mA) and operate with a wide range of supply voltages (typically 3 to 18 volts). The CMOS model number will have a C in the middle of it, e.g., the 74C04 is the CMOS equivalent to the TTL 7404. A bigh drawback is extreme sensitivity to static electricity - they must be carefully protected from static discharges.

NMOS and PMOS Logic ICs

P- and N-channel Metal Oxide Semiconductors (PMOS and NMOS) offer the advantage of higher component density than TTL chips. There are not nearly so many TTL equivalents (CMOS family does much better here). They are sensitive to damage from electrical discharge.

Component Designations

Integrated circuits in the TTL logic family have part numbers which are four to five digit numbers. With the introduction of other types of construction of devices, letters were added to center of the numbers to remind the user that basic TTL chips are not being used. The device numbers begin with a prefix which tells about its series followed by another number which identifies the individual device.