The 7400 series of TTL logic integrated circuits was introduced in October 1966 and they son became the standard for all logic integrated circuits.
The 7400 series logic TTL chips spawned a series of other derivative logic families offering slightly different characteristics: high speed, low power, etc. However the standard parameters remained the same: logic function ( a 7416 and a 74LS16 had the same function; they were pin compatible, etc.
The 7400 series TTL chips remained in use for many years. They have long been superseded by other 74xx00 logic families, but they have been so successful that the basic concept has remained the same.
7400 series main features
Some of the main or highlight features and specifications for the 7400 series logic family are detailed below:
|Summary of 7400 Series TTL Key Parameters|
|Supply voltage||Nominal 5V (4.75 - 5.25)|
|Max toggle speed||25 MHz|
|Propagation delay per gate||Typically 10 ns|
|Power consumption per gate||10 mW|
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.
From the circuit diagram the basic blocks of this simple 7400 series IC can be easily seen. The input stage of the NAND gate is a multi-emitter transistor. This takes the inputs and provides the required logic.
The next stage provides the required phase and drive for the final stage which is the standard totem pole output.
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