06-10-2012, 04:01 PM
TTL
[attachment=35545]
Transistor–transistor logic
A Motorola 68000-based computer with various TTL chips mounted on protoboards.
Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT) and resistors. It is called transistor–transistor logic because both the logic gating function (e.g., AND) and the amplifying function are performed by transistors (contrast this with RTL and DTL).
TTL is notable for being a widespread integrated circuit (IC) family used in many applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, synthesizers, etc. The designation TTL is sometimes used to mean TTL-compatible logic levels, even when not associated directly with TTL integrated circuits, for example as a label on the inputs and outputs of electronic instruments.
History
TTL was invented in 1961 by James L. Buie of TRW, "particularly suited to the newly developing integrated circuit design technology", and it was originally named transistor-coupled transistor logic (TCTL). The first commercial integrated-circuit TTL devices were manufactured by Sylvania in 1963, called the Sylvania Universal High-Level Logic family (SUHL). The Sylvania parts were used in the controls of the Phoenix missile. TTL became popular with electronic systems designers after Texas Instruments introduced the 5400 series of ICs, with military temperature range, in 1964 and the later 7400 series, specified over a narrower range, and with inexpensive plastic packages in 1966.
The Texas Instruments 7400 family became an industry standard. Compatible parts were made by Motorola, AMD, Fairchild, Intel, Intersil, Signetics, Mullard, Siemens, SGS-Thomson and National Semiconductor, and many other companies, even in the Eastern Bloc (Soviet Union, GDR, Poland, Bulgaria). 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 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, 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.
TTL with a "totem-pole" output stage
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 same current steering idea.
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. V2 and V4 collector–emitter junctions connect V4 base-emitter junction in parallel to the series-connected V3 base-emitter and V5 anode-cathode junctions. V3 base current is deprived; the transistor 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 all are 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.
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 a voltage 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 reason of this reduction are the voltage drops across the V3 base-emitter and V5 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 level. At low input voltage, the TTL input sources current which must be absorbed by the previous stage. The maximum value of this current is about 1.6 mA for a standard TTL gate. The input source has to be low-resistive enough (< 800 Ω) so that the flowing current creates only a negligible voltage drop (< 0.8 V) across it, for the input to be considered as a logical "0". TTL inputs are sometimes simply left floating to provide a logical "1", though this usage is not recommended.
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 2.2 V and 5 V (precise logic levels vary slightly between sub-types and by temperature). TTL outputs are typically restricted to narrower limits of between 0 V and 0.4 V for a "low" and between 2.6 V and 5 V for a "high", providing 0.4V of noise immunity. Standardization of the TTL levels was so ubiquitous that complex circuit boards often contained 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 was possible with chips manufactured years (sometimes over a decade) later than original components. Within usefully broad limits, logic gates could be treated as ideal Boolean devices without concern for electrical limitations.
[attachment=35545]
Transistor–transistor logic
A Motorola 68000-based computer with various TTL chips mounted on protoboards.
Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT) and resistors. It is called transistor–transistor logic because both the logic gating function (e.g., AND) and the amplifying function are performed by transistors (contrast this with RTL and DTL).
TTL is notable for being a widespread integrated circuit (IC) family used in many applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, synthesizers, etc. The designation TTL is sometimes used to mean TTL-compatible logic levels, even when not associated directly with TTL integrated circuits, for example as a label on the inputs and outputs of electronic instruments.
History
TTL was invented in 1961 by James L. Buie of TRW, "particularly suited to the newly developing integrated circuit design technology", and it was originally named transistor-coupled transistor logic (TCTL). The first commercial integrated-circuit TTL devices were manufactured by Sylvania in 1963, called the Sylvania Universal High-Level Logic family (SUHL). The Sylvania parts were used in the controls of the Phoenix missile. TTL became popular with electronic systems designers after Texas Instruments introduced the 5400 series of ICs, with military temperature range, in 1964 and the later 7400 series, specified over a narrower range, and with inexpensive plastic packages in 1966.
The Texas Instruments 7400 family became an industry standard. Compatible parts were made by Motorola, AMD, Fairchild, Intel, Intersil, Signetics, Mullard, Siemens, SGS-Thomson and National Semiconductor, and many other companies, even in the Eastern Bloc (Soviet Union, GDR, Poland, Bulgaria). 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 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, 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.
TTL with a "totem-pole" output stage
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 same current steering idea.
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. V2 and V4 collector–emitter junctions connect V4 base-emitter junction in parallel to the series-connected V3 base-emitter and V5 anode-cathode junctions. V3 base current is deprived; the transistor 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 all are 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.
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 a voltage 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 reason of this reduction are the voltage drops across the V3 base-emitter and V5 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 level. At low input voltage, the TTL input sources current which must be absorbed by the previous stage. The maximum value of this current is about 1.6 mA for a standard TTL gate. The input source has to be low-resistive enough (< 800 Ω) so that the flowing current creates only a negligible voltage drop (< 0.8 V) across it, for the input to be considered as a logical "0". TTL inputs are sometimes simply left floating to provide a logical "1", though this usage is not recommended.
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 2.2 V and 5 V (precise logic levels vary slightly between sub-types and by temperature). TTL outputs are typically restricted to narrower limits of between 0 V and 0.4 V for a "low" and between 2.6 V and 5 V for a "high", providing 0.4V of noise immunity. Standardization of the TTL levels was so ubiquitous that complex circuit boards often contained 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 was possible with chips manufactured years (sometimes over a decade) later than original components. Within usefully broad limits, logic gates could be treated as ideal Boolean devices without concern for electrical limitations.