Transistor- Transistor Logic Devices (TTL) Essay.
In 1960 Fairchild Semiconductor released its Micro-logic resistor-transistor logic (RTL) family. By 1961, both Fairchild and TI were making available off-the-shelf logic circuits. By 1964, a higher-speed logic structure, diode-transistor logic (DTL), was created to build a wide array of logic functions. Meanwhile, other companies developed a form of transistor-coupled logic. Better known as transistor-transistor logic (TTL), it first commercially appeared in 1963 in the form of a logic building-block family developed by Sylvania—SUHL for Sylvania universal high-level logic. Many companies jumped on the bandwagon.
Over the next decade and a half, they created thousands of industry-standard circuits based on TTL.
For the highest-performance applications, Fairchild, Motorola and others also developed families of emitter-coupled logic (ECL) circuits that reached multi-hundred-megahertz speeds. While working in bipolar technology moved forward, researchers experimented with metal-oxide structures. In the late ’50s, the MOSFET was born. With the development of MOS transistors, researchers took about five years to create proof-of-concept logic circuits. In 1962, RCA crafted a 16-transistor multipurpose logic circuit.
By 1965, companies could turn p-channel MOS technology into a production technology that could integrate about 1000 elements on a chip.
Not until the development of complementary metal-gate MOS technology in the late ’60s did generic standard logic circuits start to appear. The RCA CD4000 series was among the first standard logic families based on CMOS (CMOS stands for complementary metal-oxide-semiconductor, used for the manufacture of a major class of a microelectronic computer circuit incorporated into a chip or semiconductor). It offered functional equivalents to many popular TTL functions at power-consumption levels of about one-fifth to one-tenth those of the TTL parts, albeit at slightly lower operating frequencies.
But bipolar TTL still ruled the roost. Through the 70s, TTL variations came at a fast and furious pace. A high-noise-immunity version (HNIL) was developed, such as TI’s 54/74 H series. Faster versions of TTL using Schottky diodes then emerged. Called the 54/74 S series, these let the logic operate at speeds of well over 100 MHz. By the early ’80s, a veritable alphabet soup of logic family variants had been released, and designers needed a scorecard to help determine which family best fit each application. Operating voltage levels also started to shrink in the mid-’80s, with a 3.3-V standard starting to build some momentum.
Today, bus-interface logic is the “TTL” of the new millennium. Device propagation delays have shrunk from the 8 to 10 ns of the TTL days to the sub-3 ns required by the 100-MHz and faster buses used in today’s systems. Future systems will have faster buses with even lower voltages of as little as 0.9 V. This will demand another generation of bus-interface circuits. Some of those buses, though, will be serial rather than parallel, giving rise to another class of commodity circuit, the serializer/deserializer (SERDES). It takes in parallel data on one end and delivers a 2.5-Gbit/s data stream on the other end, or vice versa.
Low-voltage differential signaling (LVDS) has been employed for several years as a point-to-point connection, permitting data-transfer rates of up to 600 Mbits/s. Higher-speed and multi-drop bus-oriented versions of LVDS are starting to appear. (Bigelow 2005: n. p.): “TTL refers to the technology for designing and fabricating digital integrated circuits that employ logic gates consisting primarily of bipolar transistors.
The physical construction of integrated circuits made it more effective to replace all the input diodes in a DTL gate with a transistor, built with multiple emitters. The result is transistor-transistor logic, which became the standard logic circuit in most applications for a number of years.” It overcomes the main problem associated with DTL, i.e., lack of speed. This digital circuit is composed of bipolar transistors wired in a certain manner. Widely used since the early days of digital circuitry, a “TTL” designation on an input or output port of a device indicates that it is a digital circuit in contrast to an analog circuit.
Transistor-transistor logic (TTL) 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.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.
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 commerical 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
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 rapidly 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”). Today many of TTL gates are available (see Appendix).
All standardized common TTL circuits operate with a 5V 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.
Regarding the use of TTL in practice it became popular with electronic systems designers after “Texas Instruments” with their 1962 introduction of the 7400 series of ICs, which has 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.
Speaking about CMOS as an analog, this technology differs from TTL in a number of respects. One difference is the output signal of a CMOS device under no load condition is generally close to rail-to-rail (nearly from ground to the supply voltage) and the TTL device rises to about 3.5 volts at best. The CMOS threshold voltage is approximately at 50 percent of the supply voltage unless the device is designed for a special threshold voltage. As CMOS technology advanced into higher frequencies and lower bias voltages, there was a need to have a version of CMOS devices available which were TTL compatible. They needed to have a threshold voltage at 1.4 volts and needed to have a smaller voltage swing required at the input.
This device had very similar loading to standard CMOS devices. This has held true for both 5 volt applications as well as 3 (3.3) volt applications. 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. Several manufacturers now supply CMOS logic equivalents with TTL compatible input and output levels, usually bearing part numbers similar to the equivalent TTL component. But compared to contemporary ECL circuits, TTL uses less power and has easier design rules, but is typically slower; designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy.
To make a general conclusion concerning use of TTL today it’s necessary to say that 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 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.
- Millman, Jacob. 1979. Microelectronics Digital and Analog Circuits and System. McGraw-Hill Book Company, New York.
- Horowitz Paul and Hill Winfield. 1989. The Art of Electronics 2nd Ed. Cambridge University Press, Cambridge.
- Lesurf, Jim, University of St. Andrews. Transistor-Transistor Logic. ttp://www.st-andrews.ac.uk/~www_pa/Scots_Guide/datasheets/logic/TTL.html (29 Oct.2005).
- Wikibooks. 2005. Electronics: TTL. http://en.wikibooks.org/wiki/Electronics:TTL (28 Oct.2005).
- Bigelow, Ken. 2005. Inside Logic Gates. http://www.play-hookey.com/digital/electronics/ (28 Oct.2005).