Electronic gates A
functionally complete logic system may be composed of
relays,
valves (vacuum tubes), or
transistors. Electronic logic gates differ significantly from their relay-and-switch equivalents. They are much faster, consume much less power, and are much smaller (all by a factor of a million or more in most cases). Also, there is a fundamental structural difference. The switch circuit creates a continuous metallic path for current to flow (in either direction) between its input and its output. The semiconductor logic gate, on the other hand, acts as a high-
gain voltage amplifier, which sinks a tiny current at its input and produces a low-impedance voltage at its output. It is not possible for current to flow between the output and the input of a semiconductor logic gate. For small-scale logic, designers now use prefabricated logic gates from families of devices such as the
TTL 7400 series by
Texas Instruments, the
CMOS 4000 series by
RCA, and their more recent descendants. Increasingly, these fixed-function logic gates are being replaced by
programmable logic devices, which allow designers to pack many mixed logic gates into a single integrated circuit. The field-programmable nature of
programmable logic devices such as
FPGAs has reduced the "hard" property of hardware; it is now possible to change the logic design of a hardware system by reprogramming some of its components, thus allowing the features or function of a hardware implementation of a logic system to be changed. An important advantage of standardized integrated circuit logic families, such as the 7400 and 4000 families, is that they can be cascaded. This means that the output of one gate can be wired to the inputs of one or several other gates, and so on. Systems with varying degrees of complexity can be built without great concern of the designer for the internal workings of the gates, provided the limitations of each integrated circuit are considered. The output of one gate can only drive a finite number of inputs to other gates, a number called the "
fan-out limit". Also, there is always a delay, called the "
propagation delay", from a change in input of a gate to the corresponding change in its output. When gates are cascaded, the total propagation delay is approximately the sum of the individual delays, an effect which can become a problem in high-speed
synchronous circuits. Additional delay can be caused when many inputs are connected to an output, due to the distributed
capacitance of all the inputs and wiring and the finite amount of current that each output can provide.
Logic families There are several
logic families with different characteristics (power consumption, speed, cost, size) such as:
RDL (resistor–diode logic),
RTL (resistor–transistor logic),
DTL (diode–transistor logic),
TTL (transistor–transistor logic) and CMOS. There are also sub-variants, e.g. standard CMOS logic vs. advanced types using still CMOS technology, but with some optimizations for avoiding loss of speed due to slower PMOS transistors. The simplest family of logic gates uses
bipolar transistors, and is called
resistor–transistor logic (RTL). Unlike simple diode logic gates (which do not have a gain element), RTL gates can be cascaded indefinitely to produce more complex logic functions. RTL gates were used in early
integrated circuits. For higher speed and better density, the resistors used in RTL were replaced by diodes resulting in
diode–transistor logic (DTL).
Transistor–transistor logic (TTL) then supplanted DTL. diagram of a
NOT gate, also known as an inverter.
MOSFETs are the most common way to make logic gates. As integrated circuits became more complex, bipolar transistors were replaced with smaller
field-effect transistors (
MOSFETs); see
PMOS and
NMOS. To reduce power consumption still further, most contemporary chip implementations of digital systems now use
CMOS logic. CMOS uses complementary (both n-channel and p-channel) MOSFET devices to achieve a high speed with low power dissipation. Other types of logic gates include, but are not limited to:
Three-state logic gates A three-state logic gate is a type of logic gate that can have three different outputs: high (H), low (L) and high-impedance (Z). The high-impedance state plays no role in the logic, which is strictly binary. These devices are used on
buses of the
CPU to allow multiple chips to send data. A group of three-state outputs driving a line with a suitable control circuit is basically equivalent to a
multiplexer, which may be physically distributed over separate devices or plug-in cards. In electronics, a high output would mean the output is sourcing current from the positive power terminal (positive voltage). A low output would mean the output is sinking current to the negative power terminal (zero voltage). High impedance would mean that the output is effectively disconnected from the circuit.
Non-electronic logic gates Non-electronic implementations are varied, though few of them are used in practical applications. Many early electromechanical digital computers, such as the
Harvard Mark I, were built from
relay logic gates, using electro-mechanical
relays. Logic gates can be made using
pneumatic devices, such as the Sorteberg relay or mechanical logic gates, including on a molecular scale. Various types of fundamental logic gates have been constructed using molecules (
molecular logic gates), which are based on chemical inputs and spectroscopic outputs. Logic gates have been made out of
DNA (see
DNA nanotechnology) and used to create a computer called MAYA (see
MAYA-II). Logic gates can be made from
quantum mechanical effects, see
quantum logic gate.
Photonic logic gates use
nonlinear optical effects. In principle any method that leads to a gate that is
functionally complete (for example, either a NOR or a NAND gate) can be used to make any kind of digital logic circuit. Note that the use of 3-state logic for bus systems is not needed, and can be replaced by digital multiplexers, which can be built using only simple logic gates (such as NAND gates, NOR gates, or AND and OR gates). == See also ==