Transistor–transistor logic (TTL) is alogic family built frombipolar junction transistors (BJTs). Its name signifies that transistors perform both the logic function (the first "transistor") and the amplifying function (the second "transistor"), as opposed to earlierresistor–transistor logic (RTL) anddiode–transistor logic (DTL).
TTLintegrated circuits (ICs) were widely used in applications such ascomputers, industrial controls, test equipment and instrumentation, consumer electronics, andsynthesizers.[1]
After their introduction in integrated circuit form in 1963 bySylvania Electric Products, TTL integrated circuits were manufactured by several semiconductor companies. The7400 series byTexas Instruments became particularly popular. TTL manufacturers offered a wide range oflogic 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 TTL devices were originally made in ceramic and plasticdual in-line package(s) and in flat-pack form. Some TTL chips are now also made insurface-mount technology packages.
TTL became the foundation of computers and other digital electronics. Even afterVery-Large-Scale Integration (VLSI)CMOS integrated circuitmicroprocessors made multiple-chip processors obsolete, TTL devices still found extensive use asglue logic interfacing between more densely integrated components.
TTL was invented in 1961 byJames L. Buie ofTRW, which declared it "particularly suited to the newly developing integrated circuit design technology." The original name for TTL wastransistor-coupled transistor logic (TCTL).[2] The first commercial integrated-circuit TTL devices were manufactured by Sylvania in 1963, called the Sylvania Universal High-Level Logic family (SUHL).[3] The Sylvania parts were used in the controls of thePhoenix missile.[3] TTL became popular with electronic systems designers afterTexas Instruments introduced the 5400 series of ICs, with military temperature range, in 1964 and the later7400 series, specified over a narrower range and with inexpensive plastic packages, in 1966.[4]
The Texas Instruments 7400 family became an industry standard. Compatible parts were made byMotorola,AMD,Fairchild,Intel,Intersil,Signetics,Mullard,Siemens,SGS-Thomson,Rifa,National Semiconductor,[5][6] and many other companies, even in the Eastern Bloc (Soviet Union, GDR, Poland, Czechoslovakia, Hungary, Romania — for details see7400 series). Not only did others make compatible TTL parts, but compatible parts were made using many other circuit technologies as well. At least one manufacturer,IBM, produced non-compatible TTL circuits for its own use; IBM used the technology in theIBM System/38,IBM 4300, andIBM 3081.[7]
The term "TTL" is applied to many successive generations ofbipolar logic, with gradual improvements in speed and power consumption over about two decades. The most recently introduced family 74Fxx is still sold today (as of 2019), and was widely used into the late 90s. 74AS/ALS Advanced Schottky was introduced in 1985.[8] As of 2008, Texas Instruments continues to supply the more general-purpose chips in numerous obsolete technology families, albeit at increased prices. Typically, TTL chips integrate no more than a few hundred transistors each. Functions within a single package generally range from a fewlogic gates to a microprocessorbit-slice. TTL also became important because its low cost made digital techniques economically practical for tasks previously done by analog methods.[9]
TheKenbak-1, ancestor of the firstpersonal computers, used TTL for itsCPU instead of amicroprocessor chip, which was not available in 1971.[10] TheDatapoint 2200 from 1970 used TTL components for its CPU and was the basis for the8008 and later thex86 instruction set.[11] The 1973Xerox Alto and 1981Star workstations, which introduced thegraphical user interface, used TTL circuits integrated at the level ofarithmetic logic units (ALUs) and bitslices, respectively. Most computers used TTL-compatible "glue logic" between larger chips well into the 1990s. Until the advent ofprogrammable logic, discrete bipolar logic was used to prototype andemulatemicroarchitectures under development.
TTL inputs are the emitters of bipolar transistors. In the case of NAND inputs, the inputs are the emitters ofmultiple-emitter transistors, functionally equivalent to multiple transistors where the bases and collectors are tied together.[12] The transistor's collector is buffered by acommon emitter amplifier.
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. Unlike DTL, a small collector current (approximately 10 μA) is drawn by each of the inputs. This is because the transistor is inreverse-active mode. An approximately constant current flows from the positive rail, through the resistor and into the base of the multiple emitter transistor.[13] This current passes through the base–emitter junction of the output transistor, allowing it to conduct and pulling the output voltage low (logical zero).
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 this 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 transition the input transistor is briefly in its active region; so it draws a large current away from the base of the output transistor and thus quickly discharges its base. This is a critical advantage of TTL over DTL that speeds up the transition over a diode input structure.[14]
The main disadvantage of TTL with a simple output stage is the relatively high output resistance at outputlogical "1" that is completely determined by the output collector resistor. It limits the number of inputs that can be connected (thefanout). 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.
A common variation omits the collector resistor of the output transistor, making anopen-collector output. This allows the designer to fabricatewired logic by connecting the open-collector outputs of several logic gates together and providing a single externalpull-up resistor. If any of the logic gates becomes logic low (transistor conducting), the combined output will be low. Examples of this type of gate are the 7401[15] and 7403[16] series. Open-collector outputs of some gates have a higher maximum voltage, such as 15 V for the 7426,[17] useful when driving non-TTL loads.
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. 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 by applying the samecurrent steering idea as above.
When V2 is "off", V4 is "off" as well and V3 operates in active region as avoltage 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 series combination of V2's C-E junction and V4's B-E junction is in parallel with the series of V3 B-E, V5's anode-cathode junction, and V4 C-E. The second series combination has the higher threshold voltage, so no current flows through it, i.e. 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.[18][19]
The main advantage of TTL with a "totem-pole" output stage is the low output resistance at output logical "1". It is determined by the upper output transistor V3 operating in active region as anemitter follower. The resistor R3 does 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 (no more than 3.5 V) of the output logical "1" (even if the output is unloaded). The reasons for this reduction are the voltage drops across the V3 base–emitter and V5 anode–cathode junctions.
Like DTL, TTL is acurrent-sinking logic since a current must be drawn from inputs to bring them to a logic 0 voltage level. The driving stage must absorb up to 1.6 mA from a standard TTL input while not allowing the voltage to rise to more than 0.4 volts.[20] The output stage of the most common TTL gates is specified to function correctly when driving up to 10 standard input stages (a fanout of 10). TTL inputs are sometimes simply left floating to provide a logical "1", though this usage is not recommended.[21]
Standard TTL circuits operate with a 5-volt power supply. A TTL input signal is defined as "low" when between 0 V and 0.8 V with respect to the ground terminal, and "high" when between 2 V and VCC (5 V),[22][23] and if a voltage signal ranging between 0.8 V and 2.0 V is sent into the input of a TTL gate, there is no certain response from the gate and therefore it is considered "uncertain" (precise logic levels vary slightly between sub-types and by temperature). TTL outputs are typically restricted to narrower limits of between 0.0 V and 0.4 V for a "low" and between 2.4 V and VCC for a "high", providing at least 0.4 V ofnoise immunity. Standardization of the TTL levels is so ubiquitous that complex circuit boards often contain TTL chips made by many different manufacturers selected for availability and cost, compatibility being assured. Two circuit board units 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 later than original components. Within usefully broad limits, logic gates can be treated as ideal Boolean devices without concern for electrical limitations. The 0.4 V noise margins are adequate because of the low output impedance of the driver stage, that is, a large amount of noise power superimposed on the output is needed to drive an input into an undefined region.
In some cases (e.g., when the output of a TTL logic gate needs to be used for driving the input of a CMOS gate), the voltage level of the "totem-pole" output stage at output logical "1" can be increased closer to VCC by connecting an external resistor between the V4 collector and the positive rail. Itpulls up the V5 cathode and cuts-off the diode.[24] However, this technique actually converts the sophisticated "totem-pole" output into a simple output stage having significant output resistance when driving a high level (determined by the external resistor).
Like most integrated circuits of the period 1963–1990, commercial TTL devices are usually packaged indual in-line packages (DIPs), usually with 14 to 24 pins,[25] forthrough-hole or socket mounting. Epoxy plastic (PDIP) packages were often used for commercial temperature range components, while ceramic packages (CDIP) were used for military temperature range parts.
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 inflatpacks, a form of surface-mount package, with leads suitable for welding or soldering to printed circuit boards. Today[when?], 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 on a shared base region of the input transistor. If individually 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 ownflip chip integrated circuits with TTL; these chips were mounted on ceramic multi-chip modules.[26][27]
TTL devices consume substantially more power than equivalentCMOS devices at rest, but power consumption does not increase with clock speed as rapidly as for CMOS devices.[28] Compared to contemporaryECL circuits, TTL uses less power and has easier design rules but is substantially slower. 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. TTL is less sensitive to damage fromelectrostatic discharge than early CMOS devices.
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 not 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 current drawn from the power supply. These pulses can couple in unexpected ways between multiple integrated circuit packages, resulting in reduced noise margin and lower performance. TTL systems usually have adecoupling capacitor for every one or two IC packages, so that a current pulse from one TTL chip does not momentarily reduce the supply voltage to another.
Since the mid 1980s, several manufacturers supply CMOS logic equivalents with TTL-compatible input and output levels, usually bearing part numbers similar to the equivalent TTL component and with the samepinouts. For example, the 74HCT00 series provides many drop-in replacements for bipolar7400 series parts, but usesCMOS technology. (The "T" in "HCT" stands for "TTL-compatible". The related 74HC00 series also uses CMOS technology but is not TTL-compatible.)
Successive generations of technology produced compatible parts with improved power consumption or switching speed, or both. Although vendors uniformly marketed these various product lines as TTL withSchottky diodes, some of the underlying circuits, such as used in the LS family, could rather be consideredDTL.[29]
Variations of and successors to the basic TTL family, which has a typical gate propagation delay of 10ns and a power dissipation of 10 mW per gate, for apower–delay product (PDP) orswitching energy of about 100pJ, include:
Most manufacturers offer commercial and extended temperature ranges: for example Texas Instruments7400 series parts are rated from 0 to 70 °C, and 5400 series devices over the military-specification temperature range of −55 to +125 °C.
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.
Before the advent ofVLSI devices, TTL integrated circuits were a standard method of construction for the processors ofminicomputer and midrangemainframe computers, such as theDECVAX andData General Eclipse; however some computer families were based on proprietary components (e.g. Fairchild CTL) while supercomputers and high-end mainframes usedemitter-coupled logic. They were also used for equipment such as machine tool numerical controls, printers and video display terminals, and asmicroprocessors became more functional for "glue logic" applications, such as address decoders and bus drivers, which tie together the function blocks realized in VLSI elements. TheGigatron TTL is a more recent (2018) example of a processor built entirely with TTL integrated circuits.
While 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 anegative 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.[30] TTL inverters can also be used incrystal oscillators where their analog amplification ability is significant.
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 0.8 V to 2 V. 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 withSchmitt trigger inputs available that will reliably convert the analog input to a digital value, effectively operating as a one bit A to D converter.
TTL serial refers tosingle-endedserial communication using raw transistor voltage levels: "low" for 0 and "high" for 1.[31]UART over TTL serial is a common debug interface for embedded devices. Handheld devices such as graphing calculators andNMEA 0183-compliantGPS receivers andfishfinders also commonly use UART with TTL. TTL serial is only ade facto standard: there are no strict electrical guidelines. Driver–receiver modules interface between TTL and longer-range serial standards: one example is theMAX232, which converts from and toRS-232.[32]
Differential TTL is TTL serial carried over adifferential pair with complement levels, providing much enhanced noise tolerance. BothRS-422 andRS-485 signals can be produced using TTL levels.[33]
ccTalk is based on TTL voltage levels.
[74-series] devices are usually encapsulated in a plastic 14-pin, 16-pin, or 24-pin dual-in-line package (DIP)
