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TTL is the abbreviation for transistor-transistor logic, ie "transistor-transistor logic." Is a logic family or what is the same, a construction technology of digital electronic circuits. In components manufactured with the technology TTL input and output elements of the device are bipolar transistors.

Features:

• Its characteristic voltage is comprised between 4.75 v and 5.25 V (as seen a very narrow range.)
• logic levels are defined by the voltage range between 0.2 V and 0.8 V for the state L (low) and 2.4 V and Vcc for the state H (high).
• transmission speed between logic states is the best base, but this feature makes increasing their consumption and its main enemy. Why different versions have appeared as FAST TTL, LS, S, etc and lately the CMOS: HC, HCT and HCTLS. In some cases it may achieve little more than the 250 MHz
• TTL output signals degrade quickly if not transmitted through additional transmission circuit (can not travel more than 2 m cable without serious losses).

Historical Review:

TTL

While technology has its origins in studies of Sylvania, was Signetics the company that popularized by its greater speed and immunity to DTL noise than its predecessor, offered by Fairchild Semiconductor and Texas Instruments, mainly. Texas Instruments TTL immediately began to manufacture, 74XX and his family would become an industry standard.

Families

TTL:

technology

TTL circuits are normally prefixed with the number 74 (54 military and industrial series). Then a code of one or more digits representing the family and later one of 2 to 4 with the circuit model.

As families can be distinguished:

TTL: TTL Standard Series

-L (low power): A series of low

TTL-S (Schottky) FAST (using Schottky diodes) TTL-

AS (advanced schottky): Improved version of the previous series

TTL-LS (low power Schottky) Combining technologies L and S (the family is more widespread)

TTL-ALS (advanced low power schottky): Improved version of the AS series

TTL-F (FAST: Fairchild advanced Schottky) TTL-AF

( advanced FAST): Improved version of the F-HC

TTL (high speed CMOS): Actually it is not technology but CMOS TTL bipolar

-HCT TTL (high speed C-MOS): HC Series equipped with logic levels

TTL compatible TTL-G (GHz C-MOS) GHz (From PotatoSemi)

Versions:

A initial family 7400, or 74N, soon added a slower version but low consumption, and its counterpart 74L rapid, 74H, which was the basis of transistors doped with gold to produce recombination centers and reduce the average life of minority carriers in the base. But the problem of speed comes from a family that is saturated, ie cutting pass transistors into saturation. But a saturated transistor contains an excess of charge at the base to be removed before it starts to cut, extending their response time. The saturation state is characterized by the collector to less stress than the base. Then a diode between base and collector divert excess power by preventing the introduction of excessive loads on the base. Due to their low forward voltage using Schottky barrier diodes. So you have the 74S and 74LS families, Schottky and low power Schottky. The 74S and 74LS completely displaced 74L and 74H, because of its superior product delay · Consumption. Improvements in the manufacturing process led to the shrinking of transistors allowed the development of three new families: 74F (FAST: Fairchild Advanced Schottky Technology) Fairchild and 74AS (Advanced Schottky) and 74ALS (Advanced Low Power Schottky) of Texas Instruments. Subsequently, National Semiconductor 74F redefined for the case of buffers and interfaces, becoming 74F (r).

Technology:

TTL technology is characterized by three stages, the first of which he is appointed:

emitter input stage. Multiemisor transistor is used instead of the diode array DTL.

phase separator. It is a common emitter connected transistor which produces at its collector and emitter signals in counterphase.

Driver. It consists of several transistors, separate into two groups. The first is connected to the emitter of the phase separator and drain current to produce the low level output. The second group is connected to the collector phase splitter and produces the high level.

general

This configuration differs slightly between devices of each family, especially the output stage, which depends on whether they are buffers or not and if they are open collector, three states (ThreeState), etc. Major variations among the different families: 74N, 74L and 74H differ primarily in the value of polarization resistance, but most 74LS (not 74S) lack the characteristic multiemisor TTL transistor. Instead, they have a matrix of Schottky diodes (as DTL). This allows them to accept a wider range of input voltages up to 15V on some devices, for easy interface with CMOS. It is also quite common in circuits connected to the bus, place a pnp transistor to the input of each line, to reduce the input current and thus charge less for the bus. There are interface devices that integrate adaptation bus impedance to reduce reflections or increase speed.

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