transistor–transistor logic a motorola 68000 based computer with various ttl chips mounted on breadboards transistortransistor 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 resistortransistor logic rtl and diodetransistor 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 ttlcompatible logic levels are not associated directly with ttl integrated circuits for example as a label on the inputs and outputs of electronic instruments after their introduction in integrated circuit form in 1963 by sylvania 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 flipflops 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ttl circuit design offered higher speed or lower power dissipation to allow design optimization ttl devices were originally made in ceramic and plastic dualinline dip packages and flatpack form ttl chips are now also made in surfacemount packages ttl became the foundation of computers and other digital electronics even after larger scale integrated circuits made multiplecircuitboard processors obsolete ttl devices still found extensive use as the glue logic interfacing between more densely integrated components history a realtime clock built of ttl chips around 1979 ttl was invented in 1961 by james l buie of trw which declared it particularly suited to the newly developing integrated circuit design technology the original name for ttl was transistorcoupled transistor logic tctl the texas instruments 7400 family became an industry standard compatible parts were made by motorola amd fairchild intel intersil signetics mullard siemens sgsthomson rifa national semiconductor 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 the kenbak1 ancestor of the first personal computers used ttl for its cpu instead of a microprocessor chip which was not available in 1971 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 ttlcompatible 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 twoinput ttl nand gate with a simple output stage simplified ttl inputs are the emitters of bipolar transistors in the case of nand inputs the inputs are the emitters of multipleemitter transistors functionally equivalent to multiple transistors where the bases and collectors are tied together the output is buffered by a common emitter amplifier inputs both logical ones when all the inputs are held at high voltage the baseemitter junctions of the multipleemitter transistor are reversebiased unlike dtl a small collector current approximately 10a is drawn by each of the inputs this is because the transistor is in reverseactive mode an approximately constant current flows from the positive rail through the resistor and into the base of the multiple emitter transistor this current passes through the baseemitter junction of the output transistor allowing it to conduct and pulling the output voltage low logical zero an input logical zero note that the basecollector junction of the multipleemitter transistor and the baseemitter junction of the output transistor are in series between the bottom of the resistor and ground if one input voltage becomes zero the corresponding baseemitter junction of the multipleemitter transistor is in parallel with these two junctions a phenomenon called current steering means that when two voltagestable 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 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 a common variation omits the collector resistor of the output transistor making an opencollector output this allows the designer to fabricate logic by connecting the opencollector outputs of several logic gates together and providing a single external pullup 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 useful when driving other than ttl loads ttl with a totempole output stage standard ttl nand with a totempole output stage one of four in 7400 to solve the problem with the high output resistance of the simple output stage the second schematic adds to this a totempole pushpull output it consists of the two npn transistors v3 and v4 the lifting diode v5 and the currentlimiting resistor r3 see the figure on the right it is driven by 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 currentsteering effect the series combination of v2s ce junction and v4s be junction is in parallel with the series of v3 be v5s anodecathode junction and v4 ce the second series combination has the higher threshold voltage so no current flows through it ie 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 pullup and pulldown resistors from the output stage the main advantage of ttl with a totempole 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 an emitter 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 totempole output stage is the decreased voltage level no more than 35 v of the output logical 1 even if the output is unloaded the reason of this reduction are the voltage drops across the v3 baseemitter and v5 anodecathode junctions interfacing considerations like dtl ttl is a currentsinking 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 16ma for a standard ttl gate standard ttl circuits operate with a 5volt power supply a ttl input signal is defined as low when between 0v and 08v with respect to the ground terminal and high when between 2v and vcc 5v and if a voltage signal ranging between 08v and 20v 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 subtypes and by temperature ttl outputs are typically restricted to narrower limits of between 00v and 04v for a low and between 24v and vcc for a high providing at least 04v of noise 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 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 the 04v 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 eg 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 totempole output stage at output logical 1 can be increased closer to vcc by connecting an external resistor between the v3 collector and the positive rail it pulls up the v5 cathode and cutsoff the diode however this technique actually converts the sophisticated totempole output into a simple output stage having significant output resistance when driving a high level determined by the external resistor packaging like most integrated circuits of the period 19631990 commercial ttl devices are usually packaged in dual inline packages dips usually with 14 to 24 pins for throughhole or socket mounting the dips were usually made of epoxy plastic pdip for commercialgrade parts or of ceramic cdip for militarygrade parts beamlead 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 surfacemount package with leads suitable for welding or soldering to printed circuit boards today many ttlcompatible devices are available in surfacemount packages which are available in a wider array of types than throughhole 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 own flip chip integrated circuits with ttl these chips were mounted on ceramic multichip modules comparison with other logic families logic family ttl devices consume substantially more power than equivalent cmos devices at rest but power consumption does not increase with clock speed as rapidly as for cmos devices compared to contemporary ecl 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 levelshifting devices are required between the two logic families ttl is less sensitive to damage from electrostatic 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 linedriver devices where signals need to be sent through cables ecl by virtue of its symmetric lowimpedance output structure does not have this drawback the ttl totempole 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 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 to another several manufacturers now supply cmos logic equivalents with ttlcompatible input and output levels usually bearing part numbers similar to the equivalent ttl component and with the same pinouts for example the 74hct00 series provides many dropin replacements for bipolar 7400 series parts but uses cmos technology subtypes 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 with schottky diodes some of the underlying circuits such as used in the ls family could rather be considered dtl variations of and successors to the basic ttl family which has a typical gate propagation delay of 10ns and a power dissipation of 10mw per gate for a powerdelay product pdp or switching energy of about 100 pj include lowpower ttl l which traded switching speed 33ns for a reduction in power consumption 1mw now essentially replaced by cmos logic highspeed ttl h with faster switching than standard ttl 6ns but significantly higher power dissipation 22mw schottky ttl s introduced in 1969 which used schottky diode clamps at gate inputs to prevent charge storage and improve switching time these gates operated more quickly 3ns but had higher power dissipation 19mw lowpower schottky ttl ls used the higher resistance values of lowpower ttl and the schottky diodes to provide a good combination of speed 95ns and reduced power consumption 2mw and pdp of about 20 pj probably the most common type of ttl these were used as glue logic in microcomputers essentially replacing the former h l and s subfamilies fast f and advancedschottky as variants of ls from fairchild and ti respectively circa 1985 with millerkiller circuits to speed up the lowtohigh transition these families achieved pdps of 10 pj and 4 pj respectively the lowest of all the ttl families lowvoltage ttl lvttl for 33volt power supplies and memory interfacing most manufacturers offer commercial and extended temperature ranges for example texas instruments 7400 series parts are rated from 0 to 70c and 5400 series devices over the militaryspecification temperature range of 55 to 125c special quality levels and highreliability parts are available for military and aerospace applications radiationhardened devices for example from the snj54 series are offered for space applications applications before the advent of vlsi devices ttl integrated circuits were a standard method of construction for the processors of minicomputer and mainframe processors such as the dec vax and 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 important for glue logic applications such as fast bus drivers on a motherboard which tie together the function blocks realized in vlsi elements analog applications while originally designed to handle logiclevel 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 ttl inverters can also be used in crystal 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 08v to 2v 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 analog input to a digital value effectively operating as a one bit a to d converter list of 7400 series integrated circuits positive emittercoupled logic pecl savard john j g 2018 what computers are made from quadibloc archived from the original on 20180702 retrieved 20180716 wikimedia commons has media related to ttl fairchild semiconductor an introduction to and comparison of 74hct ttl compatible cmos logic application note 368 1984 for relative esd sensitivity of ttl and cmos texas instruments logic family application notes ttl nand and and gates from lessons in electric circuits by tony kuphaldt vtelogic familiestechnologies bicmos cmos depletionload nmos logic including hmos diode logic diodetransistor logic dtl directcoupled transistor logic dctl emittercoupled logic ecl gunning transceiver logic gtl integrated injection logic i2l nmos logic pmos logic resistortransistor logic rtl transistortransistor logic ttl current mode logic sourcecoupled logic cmlscl types static dynamic domino logic fourphase logic authority control gnd 40784447 