7400 Series TTL

The 7400 series of TTL chips became one of the de-facto standards for digital or logic integrated circuits.

Although introduced in the mid-1960s, the basic 7400 TTL chip family went on to form the basis of many of the families which are used to this day.Many elements of the 7400 TTL family of chips remain the same, from the numbering scheme to the basic concepts and standards.

7400 series main features

Some of the main or highlight features and specifications for the 7400 series logic family are detailed below:

7400 series TLL overview

The 7400 logic series of ICs were fabricated using bipolar transistor technology and this gave its name to the logic technology, TTL standing for transistor-transistor logic.

The 7400 series TTL ran from a nominal 5 Volt supply line and as a result of its popularity the 5 V line became standard for logic chips for many years, only changing when power restrictions and smaller feature sizes on chips as a result of higher integration and new processes forced the operating voltage down.

Although launched in the mid-1960s the 7400 series became the main standard set of logic widely used in electronic digital circuits. There were earlier TTL series of chips. Motorola launched a logic family with the trade name MTTL (Motorola Transistor Transistor Logic), and other series were launched by national Semiconductor, Fairchild and Signetics.

With their increasing popularity, the cost of the chips came down. Initial offerings cost many dollars each, but over time as production techniques matured and quantities rose vastly, some 7400 series chips could be bought for a few cents each.

7400 series output stages

There are three types of output stage that 7400 series logic may possess.

Totem pole: This output is the standard output format for 7400 series logic chips. It comprises two transistors and enables very fast switching times to be achieved.

· In this arrangement a driver transistor provides complementary voltages for the two output transistors, Q1 and Q2, which form the totem pole output arrangement. In this arrangement either Q1 or Q2 conducts dependent upon the complementary logic status of the inputs. The diode D1 ensures that Q2 is able to turn off rapidly when required.

The advantages of using the totem-pole output are threefold:

Low power consumption
Fast switching
Low output impedance

Open collector: This form of output has a single transistor with its emitter connected to 0V. In this way external loads can be connected between the output, i.e. the transistor collector and 5V. This has many applications including driving indicator lamps. However the speed of switching is much slower and dependent upon external influences.

Tri-state: This form of output has three states as the name implies. It is able to provide the high and low of a normal output. It is also possible to disable the output so it has no effect on the line being driven - in this state it is open circuit or floating. In order to be able to select this state, an additional "enable" input is required on the chip.

To achieve the tri-state situation, the internal circuitry is arranged so that both transistors in the totem-pole output can be tuned off at the same time.

Basic NAND gate circuit

The circuit of a basic 7400 series NAND gate is given below.