Transistor–transistor logic (TTL) is a logic family built from bipolar 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 earlier 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.
After their introduction in integrated circuit form in 1963 by Sylvania Electric Products, TTL integrated circuits were manufactured by several semiconductor companies. The 7400 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 TTL devices were originally made in ceramic and plastic dual in-line package(s) and in flat-pack form. Some TTL chips are now also made in surface-mount technology packages.
TTL became the foundation of computers and other digital electronics. Even after Very-Large-Scale Integration (VLSI) CMOS integrated circuit microprocessors made multiple-chip processors obsolete, TTL devices still found extensive use as glue logic interfacing between more densely integrated components.
History
Early transistor-coupled logic circuits were described in 1962 by R. H. Beeson and H. W. Ruegg of Fairchild Semiconductor in “New Forms of All-Transistor Logic,” presented at the IEEE International Solid-State Circuits Conference, where the approach was explicitly referred to as transistor–transistor logic (TTL or T²L). Around the same time, at Pacific Semiconductor (later acquired by TRW), James L. Buie described similar circuits as transistor-coupled transistor logic (TCTL), distinguishing them from diode-coupled transistor logic (DCTL).
As later summarized by the Computer History Museum, these developments occurred in parallel at multiple companies during the early 1960s. In 1962, Gordon Moore, then at Fairchild, referred to this approach as T²L and noted that although it was simple and well suited to integration, it suffered from reduced noise immunity and “current hogging,” limiting fan-in and fan-out; adding the components needed to correct these shortcomings reduced its advantages. Fairchild subsequently pursued diode–transistor logic as its next production family.
By the mid-1960s, a wide variety of logic families were in use, including RTL, RCTL, DCTL, LLL, CML, and DTL, with TTL emerging as an additional family.
Work at Sylvania under Tom Longo refined transistor-coupled logic into a commercially supported family, introduced in 1963 as the Sylvania Universal High-Level Logic (SUHL). Patents for the design were applied for in May and June 1963. The Sylvania parts were used in applications including the controls of the Phoenix missile.
Texas Instruments later standardized and popularized TTL with the 5400 series in 1964 and the 7400 series in 1966. In later oral history, Tom Longo stated that Texas Instruments’ early 54-series implementation closely followed the circuit configuration he had described publicly in 1963, while differentiating its products through relocating the power supply pins to the package corners, together with packaging, cost reduction, and manufacturing scale. The Texas Instruments 7400 family became a de facto industry standard. Compatible parts were manufactured by numerous semiconductor companies, including Motorola, AMD, Fairchild, Intel, National Semiconductor, Signetics, Siemens, and others, including manufacturers in the Eastern Bloc (Soviet Union, GDR, Poland, Czechoslovakia, Hungary, and Romania). In addition to bipolar implementations, compatible 7400-series parts were later produced using other circuit technologies. At least one major manufacturer, IBM, produced non-compatible TTL circuits for internal use; IBM employed TTL technology in systems such as the IBM System/38, IBM 4300, and IBM 3081.
The term "TTL" is applied to many successive generations of bipolar 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. 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 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.
The Kenbak-1, ancestor of the first personal computers, used TTL for its CPU instead of a microprocessor chip, which was not available in 1971. The Datapoint 2200 from 1970 used TTL components for its CPU and was the basis for the 8008 and later the x86 instruction set. The 1973 Xerox Alto and 1981 Star workstations, which introduced the graphical user interface, used TTL circuits integrated at the level of arithmetic logic units (ALUs) and bitslices, respectively. Most computers used TTL-compatible "glue logic" between larger chips well into the 1990s. Until the advent of programmable logic, discrete bipolar logic was used to prototype and emulate microarchitectures under development.
Implementation
Fundamental TTL gate
The basic TTL gate is a NAND structure implemented with a multiple-emitter input transistor followed by one or more amplifying stages. In a two-input TTL NAND gate, each input corresponds to one emitter of the multiple-emitter transistor VT1. The output is taken from the collector of the following transistor VT2.
All inputs high (logical 1). When both inputs are high, the emitter–base junctions of VT1 are reverse-biased. VT1 operates in its reverse-active mode. The bias resistor supplies current into the base of VT1; because the reverse current gain of VT1 is small, little current flows through its emitters. Consequently, the base current of VT1 flows almost entirely out through its collector. This collector current becomes the base current of VT2, driving VT2 into conduction and forcing the output low (logical 0).
One input low (logical 0). If either input is driven low, the corresponding emitter–base junction of VT1 becomes forward-biased. VT1 then operates in its normal active mode, and base current is diverted through the low input rather than through the collector. With little or no collector current available to drive VT2, transistor VT2 turns off and the output rises to a high level (logical 1).
Unlike diode–transistor logic (DTL), where the input network consists only of diodes, the TTL input transistor VT1 actively participates in switching. During transitions, VT1 can remove stored charge from VT2, reducing storage time and contributing to the higher switching speed of TTL compared with DTL.
The main disadvantage of TTL with a simple output stage is the relatively high output resistance at output logical "1" that is completely determined by the output collector resistor. 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. Open collector wired logic
A common variation omits the collector resistor of the output transistor, making an open-collector output. This allows the designer to fabricate wired 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 conducting), the combined output will be low. Examples of this type of gate are the 7401Quadruple 2-Input Positive-NAND Gates With Open-Collector Outputs and 7403Quadruple 2-Input Positive-NAND Gates With Open-Collector Outputs series. Open-collector outputs of some gates have a higher maximum voltage, such as 15 V for the 7426,Quadruple 2-Input High-Voltage Interface Positive-NAND Gates useful when driving non-TTL loads.
And-Or-Invert (AOI) implementation in TTL (7450 example) TTL families also include complex gates implemented directly in transistor form rather than by cascading simpler gates. An example is the 7450 dual 2-wide, 2-input AND-OR-INVERT (AOI) gate. Each product term is formed by a multiple-emitter input transistor. In the 7450, one transistor implements the term A·B and another implements C·D. Each drives its own phase-splitter transistor. The collectors of the phase-splitter transistors are connected together, and their emitters are likewise connected together. These shared nodes perform the OR function: if either product term is asserted, the corresponding phase splitter conducts and forces the shared nodes to switch state. The output stage inverts this result, yielding
Structures of this type are fundamental in digital logic design because many combinational functions are naturally expressed as sums of products. Implementing the AND-OR-INVERT function directly reduces gate count and propagation delay compared with cascading separate NAND and NOR stages, and such complex gates became common building blocks in MSI logic families. Totem-pole output stage In standard TTL families, the phase-splitter stage drives a totem-pole (push–pull) output consisting of an active pull-down transistor and an emitter-follower pull-up transistor, together with a steering diode that limits simultaneous conduction during switching. When the pull-down transistor is driven into saturation, the output is low. When it is cut off, the upper transistor operates as an emitter follower, producing the high output level at low impedance. The diode D1 and series resistor R4 limit transient current during switching and reduce shoot-through current between the supply rails.
A disadvantage of the "totem-pole" output stage is the decreased voltage level (typically 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 Q5 base–emitter and D1 anode–cathode junctions. Interfacing considerations Like DTL, TTL is a current-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.SN7400 datasheet - Texas Instruments 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.