Advantages of Resistor-Transistor Logic (RTL), Diode-Transistor Logic (DTL) and Transistor-Transistor Logic. Debashis Paul B.Tech-MBA(int)ECE Section C6803, Roll no. 32 Regn no. 10803129 Lovely Professional University, Phagwara, Punjab (India) Abst Abstrac ractt- This This is abst abstra ract ct of th thee term term pape paperr on th thee topi topicc advantages of RTL, DTL and TTL.
I. INTRODUCTION Resistor-Tr Resistor-Transis ansistor tor Logic (RTL), Diode-Trans Diode-Transistor istor logic (DTL) and Transistor Transistor Logic (TTL) is the member of Logic Logic famili families. es. Logic Logic familie familiess of digita digitall integr integrate ated d circui circuitt devices is a group of electronic logic gates gates constructed using differ different ent design designss usuall usually y with with compat compatible ible logic logic labels labels and voltag voltagee charac character terist istics ics within within a family family.. These These may also also be referred as the set of techniques used to implement logic within memory, complex integrated circuits. The first two member viz. RTL and DTL [2] have only histor historica icall signif significa icance nce since since they they are no longer longer used used in the design of digital systems. Rtl was the first commercial family to have been extensively used. It is important in the present discussion for explaining the basic operation of digital gates. Dtl circuits have been replaced by TTL. Above that, TTl is a modification of the DTL gate. So TTL gates would be easier to understand if the DTL gate is thoroughly analyzed. The basic circuit in each IC digital logic family is either a NAND or NOR gate. This basic circuit is the primary building block from which all other more complex digital components are obtained. II. REGISTOR-TRANSISTOR LOGIC (RTL)
A. Construction Resistor–t Resistor–transi ransistor stor logic (RTL) is a class class of digital digital circuits circuits built built using resistors resistors as the input network network and bipolar junction transistors transistors (BJTs) as switching switching devices. devices. RTL is the earliest earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic (DTL) and Transistor-Transistor
Fig. 1. Schematic Schematic of basic basic two input RTL RTL NOR gate
Logic (TTL). It is the basic of all other digital circuits hence a proper knowledge of its working and principle will be of great help in analyzing other logic circuits of the families. It is a logic circuit consisting of resistors and transistors. transistors. It basically performs the NOR gate function. Here we will only be applying one of two voltages to the input: 0 volts (logic 0) or +V volts (logic 1). The exact voltage used as +V depends on the circuit design parameters; the usual voltage is +3.6v. We'll assume an ordinary NPN transistor here, with a reasonable dc current gain, an emitter-base forward voltage of 0.65 volt, and a collector-emitter saturation voltage no higher than 0.3 volt. In standa standard rd RTL ICs, the base base resist resistor or is 470 and the the collecto collector r resi resist stor or is 640 640 .
B. Working When the input voltage is zero volts (actually, anything under 0.5 volt), there is no forward bias to the emitter-base junction, and the transistor does not conduct. Therefore no current flows through the collector resistor, and the output voltage is +V volts. Hence, a logic 0 input results in in a logic 1 output. When the input voltage is +V volts, the transistor's emitter base junction will clearly be forward biased. For those who like the the math mathem emat atic ics, s, we'l we'lll assu assume me a simi simila larr outpu outputt circ circui uitt
connected to this input. Thus, we'll have a voltage of 3.6 - 0.65 = 2.95 volts applied across a series combination of a 640 output resistor and a 470 input resistor. This gives us a base current of: 2.95v / 1110 = 0.0026576577 amperes = 2.66 ma
employ both diodes and transistors. DTL offers better noise margins and greater fan-outs than RTL, but suffers from low speed, especially in comparison to TTL. RTL allows the construction of NOR gates easily, but NAND gates are relatively more difficult to get from RTL. DTL, however, allows the construction of simple NAND gates from a single transistor, with the help of several diodes and resistors.
Standard fan-out for RTL gates is rated at 16. However, the fan-in for a standard RTL gate input is 3. Thus, a gate can B. Working. produce 16 units of drive current from the output, but requires 3 units to drive an input. There are low-power versions of these It consists of a single transistor working as an invertors, gates that increase the values of the base and collector resistors driven by a current that depends on the inputs to the three to 1.5K and 3.6K, respectively. Such gates demand less diodes D1-D3. The current through diodes D A and DB will only current, and typically have a fan-in of 1 and a fan-out of 2 or 3. be large enough to drive the transistor into saturation and bring They also have reduced frequency response, so they cannot the output voltage V 0 to logic “0” if all the input diodes D 1 –D3 operate as rapidly as the standard gates. To get greater output are “off”, which is true when all the inputs in the logic are “1”. drive capabilities, buffers are used. These are typically This is because when D 1-D3 are not conducting, all the current inverters which have been designed with a fan-out of 80. They from Vcc through R will go through D A and DB and into the base also have a fan-in requirement of 6, since they use pairs of of the transistor, turning it on and pulling V 0 to near ground. input transistors to get increased drive. However if any of the diodes D 1-D3 gets an input voltage of logic ‘0’, it gets forward biased and starts working. The ON C. Advantages diode takes almost all the current away from the reverse biased The primary advantage of RTL technology was that it DA and DB, limiting the transistor base current. This makes the involved a minimum number of transistors, which was an the transistor to the OFF state bringing the output voltage V 0to important consideration before integrated circuit technology logic ‘1’. (that is, in circuits using discrete components), as transistors were the most expensive component to produce. Early IC logic C. Advantages. production used the same approach briefly, but quickly One advantage of DTL over RTL is its better noise margin. transitioned to higher-performance circuits such as diode– The noise margin of a logic gate for logic level '0', Δ0, is transistor logic and then transistor–transistor logic (1963), defined as the difference between the maximum input voltage since diodes and transistors were no more expensive than that it will recognize as a '0' (V il) and the maximum voltage that resistors in the IC. may be applied to it as a '0' (V ol of the driving gate connected to it). For logic level '1', the noise margin Δ1 is the difference D. Disadvantages between the minimum input voltage that may be applied to it as The obvious disadvantage of RTL is its high current dissipation a '1' (V oh of the driving gate connected to it) and when the transistor conducts to overdrive the output biasing resistor. This requires that more current be supplied to and heat be removed from RTL circuits. In contrast, TTL circuits minimize both of these r equirements. A standard integrated circuit RTL NOR gate can drive up to 3 other similar gates. Alternatively, it has enough output to drive up to 2 standard integrated circuit RTL "buffers", each of which can drive up to 25 other standard RTL NOR gates.
III. DIODE-TRANSISTOR LOGIC (DTL)
A. Construction. Diode-Transistor Logic, or DTL, refers to the technology for designing and fabricating digital circuits wherein logic gates
Fig. 2. Schematic diagram of DTL circuit
the minimum input voltage that it will recognize as a '1' (V ih). Mathematically, Δ0 = V il-Vol and Δ1 = V oh-Vih. Any noise that
causes a noise margin to be overcome will result in a '0' being erroneously read as a '1' or vice versa. In other words, noise margin is a measure of the immunity of a gate from reading an input logic level incorrectly. In a DTL circuit, the collector output of the driving transistor is separated from the base resistor of the driven transistor by several diodes. Circuit analysis would easily show that in such an arrangement, the differences between V il and Vol, and between Voh and V ih, are much larger than those exhibited by RTL gates, wherein the collector of the driving transistor is directly connected to the base resistor of the driven transistor. This is why DTL gates are known to have better noise margins than RTL gates.
D. Limitations. A problem that DTL doesn't solve is its low speed, especially when the transistor is being turned off. Another problem with DTL circuits is that it takes as much room on the IC chip to construct a diode as it does to construct a transistor. Since "real estate/surface area” is exceedingly important in ICs, it was desirable to find a way to avoid requiring large numbers of input diodes. So TTL came up. Another problem with turning off the DTL output transistor is the fact that the effective capacitance of the output needs to charge up through Rc before the output voltage rises to the final logic '1' level, which also consumes a relatively large amount of time. TTL, however, solves the speed problem of DTL elegantly.
Fig. 3. Circuit diagram of basic TTL circuit family, signal inputs and outputs of electronic equipment may be called "TTL" inputs or outputs, signifying compatibility with the voltage levels used. TTL became popular with electronic systems designers in 1962 after Texas Instruments introduced the 7400 series of ICs, which had a wide range of digital logic block functions, and Sylvania introduced a similar family of products. The Texas Instrument family became an industry standard, but TTL devices are made by Motorola, Signetics, SGS-Thomson, National Semiconductor and many other companies. TTL became important because its low-cost integrated circuits made digital techniques economically practical for tasks previously done by analog methods.
Simplified schematic of a two input TTL NAND gate. The fundamental switching action of a TTL gate is based on a multiple-emitter input transistor. This replaces the multiple input diodes of the earlier DTL, with improved speed and a reduction in chip area. The active operation of this input IV. TRANSISTOR-TRANSISTOR LOGIC (TTL) transistor removes stored charge from the output stage A. Introduction. transistors more rapidly than a comparable DTL gate, making TTL much faster in switching. A small amount of current must Transistor-Transistor Logic (TTL) [3] is a class of digital be drawn from a TTL input to ensure proper logic levels. The circuits built from bipolar junction transistors (BJT), and total current draw must be within the capacities of the resistors. It is called transistor-transistor logic because both the preceding stage, which limits the number of nodes that can be logic gating function (e.g. AND) and the amplifying function connected. are performed by transistors (contrast this with RTL and DTL). It is notable for being a widespread integrated circuit (IC) All standardized common TTL circuits operate with a 5 volt family used in many applications such as computers, industrial power supply. A TTL signal is defined as "low" or L when controls, music synthesizers, and electronic test and between 0V and 0.8V with respect to the ground terminal, and measurement instruments. Because of the wide use of this logic "high" or H when between 2V and 5V. Standardization of TTL devices was so successful that it is routine for a complex circuit board to contain chips made by many manufacturers, based on availability and cost rather than interoperability restrictions. Like most integrated circuits of the period 1960-1990, TTL devices are usually packaged in through hole, dual in line
packages with between 14 and 24 lead wires, made usually of epoxy plastic but also commonly ciramic. Other packages included the flat-pack, used for military and aerospace applications, and beam-lead chips without packages for assembly into larger arrays. As surface-mounted devices became more common through the 1990's, most popular TTL devices were made available in these packages.
TTL has a much higher speed than DTL. This is due to the fact that when the output transistor is turned off, there is a path for the stored charge in its base to dissipate through, allowing it to reach cut-off faster than a DTL output transistor. At the same time, the equivalent capacitance of the output is charged from Vcc through T3 and the output diode, allowing the output voltage to rise more quickly to logic '1' than in a DTL output wherein the output capacitance is charged through a resistor.
Before the advent of VLSI devices, TTL integrated circuits were standard method of constructions for the processors of D. Comparisons with other families. mini-computer and main frame processors; such as the Digital TTL devices consume substantially more power than Equipment Corporation VAX and Data General Eclipse, and for equipment such as machine tool numerical controls, equivalent CMOS devices at rest, but power consumption does printers, and video display terminals. As microprocessors not increase with clock speed as rapidly as for CMOS devices. became more functional, TTL devices became important for Compared to contemporary ECL circuits, TTL uses less power "glue logic" applications, such as fast bus drivers on a and has easier design rules but is substantially slower. motherboard, which tie together the function blocks realized in Designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy, but VLSI elements. level-shifting devices are required between the two logic families. TTL is less sensitive to damage from electrostatic B. Working. discharge than early CMOS devices. Applying a logic '1' input voltage to both emitter inputs of T1 reverse-biases both base-emitter junctions, causing current to Due to the output structure of TTL devices, the output flow through R1 into the base of T2, which is driven into impedance is asymmetrical between the high and low state, saturation. When T2 starts conducting, the stored base charge making them unsuitable for driving transmission lines. This of T3 dissipates through the T2 collector, driving T3 into cut- drawback is usually overcome by buffering the outputs with off. On the other hand, current flows into the base of T4, special line-driver devices where signals need to be sent causing it to saturate and pull down the output voltage Vo to through cables. ECL, by virtue of its symmetric low-impedance logic '0', or near ground. Also, since T3 is in cut-off, no output structure, does not have this drawback. current will flow from Vcc to the output, keeping it at logic '0'. The TTL "totem-pole" output structure often has a T2 always provides complementary inputs to the bases of T3 and T4, such that T3 and T4 always operate in opposite momentary overlap when both the upper and lower transistors are conducting, resulting in a substantial pulse of current drawn regions, except during momentary transition between regions. from the supply. These pulses can couple in unexpected ways Now applying a logic '0' input voltage to at least one emitter between multiple integrated circuit packages, resulting in input of T1 will forward-bias the corresponding base-emitter reduced noise margin and lower performance. TTL systems junction, causing current to flow out of that emitter. This usually have a decoupling capacitor for every one or two IC causes the stored base charge of T2 to discharge through T1, packages, so that a current pulse from one chip does not driving T2 into-cut-off. Now that T2 is in cut-off, current momentarily reduce the supply voltage to the others. from Vcc will be diverted to the base of T3 through R3, Several manufacturers now supply CMOS logic equivalents causing T3 to saturate. On the other hand, the base of T4 will be deprived of current, causing T to go into cut-off. With T4 with TTL-compatible input and output levels, usually bearing in cut-off and T3 in saturation, the output Vo is pulled up to part numbers similar to the equivalent TTL component and with the same pin outs. For example, the 74HCT00 series logic '1', or closer to Vcc. provides many drop-in replacements for bipolar 7400 series Outputs of different TTL gates that employ the totem-pole parts, but uses CMOS technology. configuration must not be connected together since differences in their output logic will cause large currents to flow from the logic '1' output to the logic '0' output, destroying both output stages. The output of a typical TTL gate under normal operation can sink currents of up to 16 mA.
C. Advantages.
TABLE I THE TTL FAMILY AND CHARACTERISTICS [1]. TTL Series Standard TTL
Prefix 74
Example 7400
Features Oldest, slowest, most power hungry member of the TTL
Low Power TTL
74L
74L00
Schottky TTL
74S
74S00
Low-Power Schottky TTL
74LS
74LS00
Advanced S TTL
74AS
74ALS00
Advanced TTL
74ALS
74ALS00
74F
74F00
Fast TTL
LS
family. preparation of this term paper by giving knowledge about the Old but subject matter and also the required format. optimized to consume less power than REFERENCES/BIBLIOGRAPHY “Standard” Old, but [1] Jain R. P., “Modern Digital Electronics”, Tata McGraw-Hill publication, optimized for 3rd ed, page 93-109, 2007 speed. Consumes too much power! rd Faster and lower [2] Morris M. Mano, “Digital Design”, Prentice-Hall publication, 3 Ed, Page power than the 399-421, 2006 above subfamilies. [3] Donald P Leach, Albert Paul Malvino, Goutam Saha, “Digital Principles Becoming rather And Applications”, The McGraw-Hill Companies publication, 6th ed, page 506obsolete 531, 1995. Very fast, but quite power [4] http://www.siliconfareast.com/dtl.htm hungry! Very good [5] http://www.play-hookey.com/digital/electronics speed-power ratio. Quite popular member [6] http://hyperphysics.phy-astr.gsu.edu/Hbase/electronic/ logfam.html of this Family Fairchild’s fast [7] http://www.asic-world.com/digital/logic1.html#Digital_ Logic Families TTL family. Excellent speed power product.
ACKNOWLEDGEMENT I take this opportunity to thank my subject instructor Mr. Jaspinder Singh Sidhu, lecturer, Dept. of ECE, LPU for allocating me the above topic and also guiding me in