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Transistor—transistor logic TTL is a logic family built from bipolar junction transistors.
Its name signifies that transistors perform both the logic function the first ‘transistor’ and the amplifying function the second ‘transistor’ ; it is the same naming convention used in resistor—transistor logic RTL and diode—transistor logic DTL.
TTL integrated circuits ICs were widely used in applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, and synthesizers. Sometimes TTL-compatible logic levels are not associated directly with TTL integrated circuits, for example as a label on the inputs and outputs of electronic instruments.
The series by Texas Instruments became particularly popular. TTL manufacturers offered a wide range of logic gates, flip-flops, counters, and other circuits. Variations of the original TTL circuit design offered higher speed or lower power dissipation to allow design optimization.
TTL devices were originally made in ceramic and plastic dual-in-line DIP packages, and flat-pack form. TTL chips are now also made in surface-mount packages. TTL became the foundation of computers and other digital electronics. Even after larger scale integrated circuits made multiple-circuit-board processors obsolete, TTL devices still found extensive use as the glue logic interfacing between more densely integrated components.
TTL was invented in by James L. Buie of TRW, which declared it, ‘particularly suited to the newly developing integrated circuit design technology. Not only did others make compatible TTL parts, but compatible parts were made using many other circuit technologies as well.
The most recently introduced family 74Fxx is still sold today, and was widely used into the late 90s. Typically, TTL chips integrate no more than a few hundred transistors each. Functions within a single package generally range from a few logic gates to a microprocessor bit-slice. TTL also became important because its low cost made digital techniques economically practical for tasks previously done by analog methods.
Most computers used TTL-compatible ‘glue logic’ between larger chips well into the s. Until the advent of programmable logic, discrete bipolar logic was used to prototype and emulate microarchitectures under development. TTL inputs are the emitters of bipolar transistors.
In the case of NAND inputs, the inputs are the emitters of multiple-emitter transistors, functionally equivalent to multiple transistors where the bases and collectors are tied together. Inputs both logical ones. When all the inputs are held at high voltage, the base—emitter junctions of the multiple-emitter transistor are reverse-biased.
This is because the transistor is in reverse-active mode. An approximately constant current flows from the positive rail, through the resistor and into the base of the multiple emitter transistor. An input logical zero.
Note that the base—collector junction of the multiple-emitter transistor and the base—emitter junction of the output transistor are in series between the bottom of the resistor and ground. If one input voltage becomes zero, the corresponding base—emitter junction of the multiple-emitter transistor is in parallel with these two junctions.
A phenomenon called current steering means that when two voltage-stable elements with different threshold voltages are connected in parallel, the current flows through the path with the smaller threshold voltage. That is, current flows out of xatasheet input and into the zero low voltage source. As a result, no current flows through the base of the output transistor, causing it to stop conducting and the output voltage becomes high logical one. During the datasyeet the input transistor is briefly in its active region; so dtaasheet draws a large datashest away from the base of the output transistor and thus quickly discharges its base.
It limits the number of inputs that can be connected the fanout. Some advantage of the simple output stage is the high voltage level up to VCC of the output logical ‘1’ when the output is not loaded.
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A common variation omits the collector resistor of the output transistor, making an open-collector output. This dahasheet the designer to fabricate logic by connecting the open-collector outputs of several logic gates together and providing a single external pull-up resistor.
If any of the logic gates becomes logic low transistor conductingthe combined output will be low. 7915ccv of this type of gate are the  and series. Open-collector outputs of some satasheet have a higher maximum voltage, such as 15V for the , useful when driving other than TTL loads. TTL with a ‘totem-pole’ output stage Standard TTL NAND with a ‘totem-pole’ output stage, one of four in To solve the problem with the high output resistance of the simple output stage the second schematic adds to this a ‘totem-pole’ ‘push—pull’ output.
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It consists of the two n-p-n transistors V3 and V4, the ‘lifting’ diode V5 and the current-limiting resistor R3 see the figure on the right. It is driven datasbeet applying the same current steering idea as above.
When V2 is ‘off’, V4 is ‘off’ as well and V3 operates in active region as a voltage follower producing high output voltage logical ‘1’. When V2 is ‘on’, it activates V4, driving low voltage logical ‘0’ to the output. Again there is a current-steering effect: The second series combination has the higher threshold voltage, so no current flows through it, i.
V3 base current is deprived. Transistor V3 turns ‘off’ and it does not impact on the output. In the middle of the transition, the resistor R3 limits the current flowing directly through the series connected transistor V3, diode V5 and transistor V4 that are all conducting. It also limits the output current in the case of output logical ‘1’ and short connection to the ground.
The strength of the gate may be increased without proportionally affecting the power consumption by removing the pull-up and pull-down resistors from the output stage. It is determined by the upper output transistor V3 operating in active region as an emitter follower. The resistor R3 dwtasheet not increase the output resistance since it is connected in the V3 collector and its influence is compensated by the negative feedback. A disadvantage of the ‘totem-pole’ output stage is the decreased voltage level datsheet more than 3.
The reason of this reduction are the voltage drops across the V3 base—emitter and V5 anode—cathode junctions. Interfacing considerationsLike DTL, TTL is a current-sinking logic since a current must be drawn from inputs to bring them to a logic 0 level.
At low input voltage, the TTL input sources current which must be absorbed by the previous stage. The maximum value of this input current is about 1. TTL outputs are typically restricted to narrower limits of between 0. Standardization of the TTL levels is so ubiquitous that complex circuit boards 7915cf contain TTL chips made by many different manufacturers selected for availability and datashert, compatibility being assured; two circuit board 795cv off the same assembly line on different successive days or weeks might have a different mix of brands of chips in the same positions on the board; repair is possible with chips manufactured years sometimes over a decade later than original components.
Within usefully broad limits, logic gates can be treated as ideal Boolean devices without concern for electrical limitations.
In some cases e. It pulls up the V5 cathode and cuts-off the diode. PackagingLike most integrated circuits of the period —, commercial TTL devices are usually packaged in dual in-line packages DIPsusually with 14 to 24 pins, for through-hole or socket mounting. Beam-lead chip dies without packages were made for assembly into larger arrays as hybrid integrated circuits. Parts for military and aerospace applications were packaged in flatpacks, a form of surface-mount package, with leads suitable for welding or soldering to printed circuit boards.
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Today, many TTL-compatible devices are available in surface-mount packages, which are available in a wider array of types than through-hole packages. TTL is particularly well suited to bipolar integrated circuits because additional inputs to a gate merely required additional emitters datasbeet a shared base region of the input transistor. If dtasheet packaged transistors were used, the cost of all the transistors would discourage one from using such an input structure.
But in an integrated circuit, the additional emitters for extra gate inputs add only a small area. At least one computer manufacturer, IBM, built its own flip chip integrated circuits with TTL; these chips were mounted on ceramic multi-chip modules. Designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy, but level-shifting devices are required between the two logic families.
Due to the output structure of TTL devices, the output impedance is asymmetrical between the high and low state, making them unsuitable for driving transmission lines.
This drawback is usually overcome by buffering the outputs with special line-driver devices where signals need to be sent through cables. ECL, by virtue of its symmetric low-impedance output structure, does datsaheet have this drawback. The TTL ‘totem-pole’ output structure often has a momentary overlap when both the upper and lower transistors are conducting, resulting in a substantial pulse of datawheet drawn from the power supply.
These pulses can couple in unexpected ways between multiple integrated circuit packages, resulting in reduced noise 7915fv and lower performance. TTL systems usually have a decoupling capacitor for every one or two IC packages, so that a current pulse from one TTL chip does not momentarily reduce the supply voltage dataseet another. Several manufacturers now supply Datashert logic equivalents with TTL-compatible input and output levels, usually bearing part numbers similar datsheet the equivalent TTL component and with the same pinouts.
Sub-typesSuccessive generations of technology produced compatible parts with improved power consumption or switching speed, or both. Although datashheet uniformly marketed these various product lines as TTL with Schottky diodes, some of the underlying circuits, such as used in the LS family, could rather be considered DTL.
Probably the most common type of TTL, these were used as glue logic in microcomputers, essentially replacing the former H, L, and S sub-families. Most manufacturers offer commercial and extended temperature ranges: Special quality levels and high-reliability parts are available for military and aerospace applications. Radiation-hardened devices for example from the SNJ54 series are offered for space applications. ApplicationsBefore 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 DEC VAX datashheet 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 datasheet for ‘glue logic’ applications, such as datashwet bus drivers on a motherboard, which tie together the function blocks realized in VLSI elements.
Analog applicationsWhile originally designed to handle logic-level digital signals, a TTL inverter can be biased as an analog amplifier. Connecting a resistor between the output and the input biases the TTL element as a negative feedback amplifier. Such amplifiers may be useful to convert analog signals to the digital domain but would not ordinarily be used where analog amplification is the primary purpose. A TTL gate may operate inadvertently as an analog amplifier if the input is connected to a slowly changing input signal that traverses the unspecified region from 0.
The output can be erratic when the input is in this range.
A slowly changing input like this can also cause excess power dissipation in the output circuit. If such an analog input must be used, there are specialized TTL parts with Schmitt trigger inputs available that will reliably convert the dwtasheet input to a digital value, effectively operating as a one bit A to D converter.
Retrieved 16 April Retrieved 17 September The true origins of the personal computer’ Archived at the Wayback Machine. McGraw-Hill Book Company, p.