Introduction to Logic Gates
A logic gate performs a logical operation on one or more logic inputs and produces a single logic output. The logic normally performed is Boolean logic and is most commonly found in digital circuits. Logic gates are primarily implemented electronically using diodes or transistors, but can also be constructed using electromagnetic relays, fluidics, optics, molecules, or even mechanical elements.
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In electronic logic, a logic level is represented by a voltage or current, (which depends on the type of electronic logic in use). Each logic gate requires power so that it can source and sink currents to achieve the correct output voltage. In logic circuit diagrams the power is not shown, but in a full electronic schematic, power connections are required.
Resistor-Transistor Logic (RTL):
Resistor-transistor logic gates use Transistors to combine multiple input signals, which also amplify and invert the resulting combined signal. Often an additional transistor is included to re-invert the output signal. This combination provides clean output signals and either inversion or non-inversion as needed.
RTL gates are almost as simple as DL gates, and remain inexpensive. They also are handy because both normal and inverted signals are often available. However, they do draw a significant amount of current from the power supply for each gate. Another limitation is that RTL gates cannot switch at the high speeds used by today’s computers, although they are still useful in slower applications.
Although they are not designed for linear operation, RTL integrated circuits are sometimes used as inexpensive small-signal amplifiers, or as interface devices between linear and digital circuits.
RTL Logic Circuit:
Resistor-transistor logic (RTL) is a class of digital circuits built using resistors as the input network and bipolar junction transistors (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit used; other classes include diode-transistor logic (DTL) and transistor-transistor logic (TTL).
Advantages of RTL Logic circuits:
The primary advantage of RTL technology was that it involved a minimum number of transistors, which was an important consideration before integrated circuit technology (that is, in circuits using discrete components), as transistors were the most expensive component to produce. Early IC logic production (such as Fairchild’s in 1961) used the same approach briefly, but quickly transitioned to higher-performance circuits such as diode-transistor logic and then transistor-transistor logic (starting 1963 at Sylvania), since diodes and transistors were no more expensive than resistors in the IC.
Limitations of RTL Logic circuits
The obvious disadvantage of RTL is its high current dissipation 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 requirements.
Lancaster says that integrated circuit RTL NOR gates (which have one transistor per input) may be constructed with “any reasonable number” of logic inputs, and gives an example of an 8-input NOR gate.
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.
Diode-Transistor Logic (DTL):
By letting diodes perform the logical AND or OR function and then amplifying the result with a transistor, we can avoid some of the limitations of RTL. DTL takes diode logic gates and adds a transistor to the output, in order to provide logic inversion and to restore the signal to full logic levels.
Diode-Transistor Logic (DTL) is a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors; it is the direct ancestor of transistor-transistor logic. It is called diode-transistor logic because the logic gating function (e.g., AND) is performed by a diode network and the amplifying function is performed by a transistor (contrast this with RTL and TTL).
Operation of DTL:
With the simplified circuit shown in the picture the negative bias voltage at the base is required to prevent unstable or invalid operation. In an integrated circuit version of the gate, two diodes replace R3 to prevent any base current when one or more inputs are at low logic level. Alternatively to increase fan-out of the gate an additional transistor and diode may be used. The IBM 1401 used DTL circuits almost identical to this simplified circuit, but solved the base bias level problem mentioned above by alternating NPN and PNP based gates operating on different power supply voltages instead of adding extra diodes.
Advantages of DTL:
One advantage of digital circuits when compared to analog circuits is that signals represented digitally can be transmitted without degradation due to noise. For example, a continuous audio signal, transmitted as a sequence of 1s and 0s, can be reconstructed without error provided the noise picked up in transmission is not enough to prevent identification of the 1s and 0s. An hour of music can be stored on a compact disc as about 6 billion binary digits.
In a digital system, a more precise representation of a signal can be obtained by using more binary digits to represent it. While this requires more digital circuits to process the signals, each digit is handled by the same kind of hardware. In an analog system, additional resolution requires fundamental improvements in the linearity and noise charactersitics of each step of the signal chain.
Computer-controlled digital systems can be controlled by software, allowing new functions to be added without changing hardware. Often this can be done outside of the factory by updating the product’s software. So, the product’s design errors can be corrected after the product is in a customer’s hands.
Information storage can be easier in digital systems than in analog ones. The noise-immunity of digital systems permits data to be stored and retrieved without degradation. In an analog system, noise from aging and wear degrade the information stored. In a digital system, as long as the total noise is below a certain level, the information can be recovered perfectly.
Disadvantages of DTL:
In some cases, digital circuits use more energy than analog circuits to accomplish the same tasks, thus producing more heat. In portable or battery-powered systems this can limit use of digital systems.
For example, battery-powered cellular telephones often use a low-power analog front-end to amplify and tune in the radio signals from the base station. However, a base station has grid power and can use power-hungry, but very flexible software radios. Such base stations can be easily reprogrammed to process the signals used in new cellular standards.
Digital circuits are sometimes more expensive, especially in small quantities.
The sensed world is analog, and signals from this world are analog quantities. For example, light, temperature, sound, electrical conductivity, electric and magnetic fields are analog. Most useful digital systems must translate from continuous analog signals to discrete digital signals. This causes quantization errors. Quantization error can be reduced if the system stores enough digital data to represent the signal to the desired degree of fidelity. The Nyquist-Shannon sampling theorem provides an important guideline as to how much digital data is needed to accurately portray a given analog signal.
In some systems, if a single piece of digital data is lost or misinterpreted, the meaning of large blocks of related data can completely change. Because of the cliff effect, it can be difficult for users to tell if a particular system is right on the edge of failure, or if it can tolerate much more noise before failing.
Digital fragility can be reduced by designing a digital system for robustness. For example, a parity bit or other error management method can be inserted into the signal path. These schemes help the system detect errors, and then either correct the errors, or at least ask for a new copy of the data. In a state-machine, the state transition logic can be designed to catch unused states and trigger a reset sequence or other error recovery routine.
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Embedded software designs that employ Immunity Aware Programming, such as the practice of filling unused program memory with interrupt instructions that point to an error recovery routine. This helps guard against failures that corrupt the microcontroller’s instruction pointer which could otherwise cause random code to be executed. Digital memory and transmission systems can use techniques such as error detection and correction to use additional data to correct any errors in transmission and storage.
On the other hand, some techniques used in digital systems make those systems more vulnerable to single-bit errors. These techniques are acceptable when the underlying bits are reliable enough that such errors are highly unlikely.
TTL Logic Circuit:
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.
*TTL contrasts with the preceding resistor-transistor logic (RTL) and diode-transistor logic (DTL) generations by using transistors not only to amplify the output but also to isolate the inputs. The p-n junction of a diode has considerable capacitance, so changing the logic level of an input connected to a diode, as in DTL, requires considerable time and energy.
As shown in the top schematic at right, the fundamental concept of TTL is to isolate the inputs by using a common-base connection, and amplify the function using a common emitter connection. Note that the base of the output transistor is driven high only by the forward-biased base-collector junction of the input transistor. The second schematic adds to this a “totem-pole output”. When V2 is off (output equals 1), the resistors turn V3 on and V4 off, resulting in a stronger 1 output. When V2 is on, it activates V4, driving 0 to the output. The diode forces the emitter of V3 to ~0.7 V, while V4 base-emitter junction and V2 collector-emitter junction pull its base to a voltage ~0.7, turning it off. By removing pull-up and pull-down resistors from the output stage, this allows the strength of the gate to be increased without proportionally affecting power consumption.
TTL is particularly well suited to integrated circuits because the inputs of a gate may all be integrated into a single base region to form a multiple-emitter transistor. Such a highly customized part might increase the cost of a circuit where each transistor is in a separate package, but, by combining several small on-chip components into one larger device, it conversely reduces the cost of implementation on an IC.
As with all bipolar logic, a small current must be drawn from a TTL input to ensure proper logic levels. The total current drawn must be within the capacities of the preceding stage, which limits the number of nodes that can be connected (the fanout).
All standardized common TTL circuits operate with a 5-volt power supply. A TTL input signal is defined as “low” when between 0V and 0.8V with respect to the ground terminal, and “high” when between 2.2V and 5V (precise logic levels vary slightly between sub-types). TTL outputs are typically restricted to narrower limits of between 0V and 0.4V for a “low” and between 2.6V and 5V 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.
Advantages of TTL Logic circuit:
Advantages of TTL logic family, one should have a basic idea about RTL, DTL etc. Diode logic (DL) uses diodes to implement logical functions like AND and OR. But the disadvantage is that it can not perform NOT operation. As AND and OR are not complete functions by themselves, they can not perform several logic functions without NOT. Hence, there was a need for some device which can perform a NOT function as diodes can not. That device is a transistor. Then came the DTL which uses a transistor along with diodes. As a transistor can act as an inverter, NAND (NOT-AND) & NOR (NOT-OR) operations can be performed. But this logic uses several diodes which will slow down its operation. Due to the delay offered by them, the logic levels may sometimes change i. e. 0 t0 1 or 1 to 0. Then came TTL. This logic uses a multi emitter transistor, a transistor with many emitter terminals. As every emitter is nothing but a diode, this logic eliminates the use of all diodes. This is the major advantage.
As transistor becomes ON and OFF much rapidly than a diode, switching time will be faster.
TTL, or Transistor-transistor logic replaced resistor-transistor logic, and used much less power. The TTL family is very fast and reliable, and newer faster, less power-consuming, etc. types are always being developed.
In TTL (Transistor-Transistor Logic), think that the device using this technology is made from several transistors. Another advantage is that many more chips employ this
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