Transistor-transistor logic

From Academic Kids

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68k_ttl.jpg
A Motorola 68000-based computer with various TTL chips.

Transistor-transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT), and resistors. It is notable for being a widespread integrated circuit (IC) family used in many applications such as computers, industrial controls, music synthesizers, and electronic test and measurement instruments.

Contents

History

TTL became popular with electronic systems designers in 1962 after Texas Instruments introduced the 7400 series of ICs, which had a wide range of digital logic block functions. The Texas Instrument family became an industry standard but TTL devices are made by Motorola, Signetics, and National Semiconductor and many other companies. TTL became important because it was the first time that low-cost integrated circuits made digital techniques economically practical for tasks previously done by analog methods.

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TTL_Clock.jpg
A real-time clock built of TTL chips designed about 1979.

Functions

Each integrated circuit performs separate building-block functions such as

Theory

TTL integrated circuits are examples of small-scale to medium-scale integration. Each "chip" contains the equivalent of a few dozen to a few hundred transistors, contrasting with early very-large-scale integration VLSI devices that had the equivalent of up to 10,000 transistors, and modern microprocessors that are equivalent to tens of millions of transistors.

The fundamental switching action of a TTL gate is based on a multiple-emitter input transistor. This replaces the multiple input diodes of the earlier DTL logic, with improved speed and a reduction in chip area. The active operation of this input transistor removes stored charge from the output stage transistors more rapdily than a comparable DTL gate, making TTL much faster in switching. A small amount of current must be drawn from a TTL input to insure proper logic levels. The total current draw must be within the capacities of the preceding stage, which limits the number of nodes that can be connected ( the "fan-out").

All standardized common TTL circuits operate with a 5 volt power supply. A TTL signal is defined as "low" or L when between 0V and 0.8V with respect to the ground terminal, and "high" or H when between 2V and 5V. Standardization of TTL devices was so successful that it is routine for a complex circuit board to contain chips manufactured by Texas Instruments, Signetics, National Semiconductor, Motorola, Hitachi, and others, based on availability and cost rather than interoperability restrictions.

Like most integrated circuits of the period 1960-1990, TTL devices are usually packaged in through-hole, dual in-line packages with between 14 and 24 lead wires, made usually of epoxy plastic but also commonly ceramic. Other packages included the flat-pack, used for military and aerospace applications, and beam-lead chips without packages for assembly into larger arrays. As surface-mounted devices became more common through the 1990's, many popular TTL devices were made available in these packages.

Comparison with other logic families

Generally, TTL devices consume more power than an equivalent CMOS device at rest, but power consumption did not increase with clock speed as rapidly as for CMOS devices. Compared to contemporary ECL circuits, TTL uses less power and had easier design rules, but was typically slower; designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy. TTL was historically considered significantly less sensitive to damage from electrostatic discharge than CMOS, although current CMOS designs now exhibit less static sensitivity than earlier designs.

Sub-types

Variations of the basic TTL logic family include:

  • Low-power TTL, which traded switching speed for a slight reduction in power consumption (now essentially supplanted by CMOS logic)
  • Schottky TTL, which used Schottky diode clamps at gate inputs to prevent charge storage and speed switching time. These gates operated more quickly but had higher power dissipation
  • Low-power Schottky - used the higher resistance values of low-power TTL and the Schottky diodes to provide a good combination of speed and reduced power consumption. Probably the most common type of TTL since these were used as glue logic in microcomputers.
  • Most manufacturers offer commercial and extended temperature ranges; for example Texas Instruments 7400-series parts are rated from 0 to 70 degrees Celsius, and 5400-series devices over the military-specification temperature range of -55 to +125 degrees Celsius.
  • Radiation-hardened devices are offered for space applications
  • Special quality levels and high-reliability parts are available for military and aerospace applications.

Several manufacturers now supply CMOS logic equivalents with TTL compatible input and output levels, usually bearing part numbers similar to the equivalent TTL component and with the same pin-out diagram.


Applications

Before the advent of VLSI devices, TTL integrated circuits were a standard method of construction for the processors of mini-computer and mainframe processors ( such as the Digital Equipment Corporation VAX and Data General Eclipse, and for equipment such as machine tool numerical controls, printers, and video display terminals. As microprocessors became more functional, TTL devices became important for "glue logic" applications, such as fast bus drivers on a motherboard, which tie together the function blocks realized in VLSI elements.

See also

References

Jacob Millman, "Microelectronics Digital and Analog Circuits and Systems", McGraw-Hill Book Company, New York, 1979 ISBN 007042327X

Paul Horowitz and Winfield Hill, "The Art of Electronics 2nd Ed. " Cambridge University Press, Cambridge, 1989 ISBN 0521370957

Don Lancaster, "TTL Cookbook", Howard W. Sams and Co., Indianapolis, 1975, ISBN 0672210355

The Engineering Staff, "The TTL Data Book for Design Engineers", 1st Ed., Texas Instruments, Dallas Texas, 1973, no ISBN

Fairchild Semiconductor, "Application Note 368 (http://www.fairchildsemi.com/an/AN/AN-368.pdf)" (for relative ESD sensitivity of TTL and CMOS)de:Transistor-Transistor-Logik es:Tecnología TTL fr:TTL ja:Transistor-transistor logic pl:Transistor-Transistor Logic ru:TTL

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