NAND gate
From Biomatics.org
| INPUT A B |
OUTPUT A NAND B |
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| 0 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
The NAND gate is a digital logic gate that behaves in a manner that corresponds to the truth table to the left. A LOW output results only if both the inputs to the gate are HIGH. If one or both inputs are LOW, a HIGH output results. The NAND gate is a universal gate in the sense that any boolean function can be implemented by NAND gates.
Digital systems employing certain logic circuits take advantage of NAND's functional completeness. In complicated logical expressions, normally written in terms of other logic functions such as AND, OR, and NOT, writing these in terms of NAND saves on cost, because implementing such circuits using NAND gate yields a more compact result than the alternatives.
NAND gates can also be made with more than two inputs, yielding an output of LOW if all of the inputs are HIGH, and an output of HIGH if any of the inputs is LOW. These kinds of gates therefore operate as n-ary operators instead of a simple binary operator. Algebraically, these can be expressed as the function NAND(a, b, ..., n), which is logically equivalent to NOT(a AND b AND ... AND n).
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Symbols
There are three symbols for NAND gates: the 'distinctive' (MIL/ANSI) symbol and the 'rectangular' IEC symbol, as well as a deprecated DIN symbol sometimes found on old schematics. For more information see logic gate symbols.
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| MIL/ANSI Symbol | IEC Symbol | DIN Symbol |
Hardware description and pinout
NAND gates are basic logic gates, and as such they are recognised in TTL and CMOS ICs.
CMOS version
The standard, 4000 series, CMOS IC is the 4011, which includes four independent, two-input, NAND gates.
Availability
These devices are available from most semiconductor manufacturers such as Fairchild Semiconductor, Philips or Texas Instruments. These are usually available in both through-hole DIL and SOIC format. Datasheets are readily available in most datasheet databases.
The standard 2-, 3-, 4- and 8-input NAND gates are available:
- CMOS
- 4011: Quad 2-input NAND gate
- 4023: Triple 3-input NAND gate
- 4012: Dual 4-input NAND gate
- 4068: Mono 8-input NAND gate
- TTL
- 7400: Quad 2-input NAND gate
- 7410: Triple 3-input NAND gate
- 7420: Dual 4-input NAND gate
- 7430: Mono 8-input NAND gate
Implementations
The NAND gate is the easiest to manufacture, and also has the property of functional completeness. That is, any other logic function (AND, OR, etc.) can be implemented using only NAND gates. An entire processor can be created using NAND gates alone.
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A New DNA computing model for the NAND gate based on induced hairpin formation
http://lib.bioinfo.pl/pmid:15527948
Living Computers within body may aid healing
Researchers at the California Institute of Technology (Caltech) have created a molecular computer that can run calculations inside a living cell. Project leaders Maung Nyan Win and Christina Smolke have revealed that, so far, they have tested the living computer on a living yeast cell. The researchers believe that future models of the computer, made from the DNA-like molecule RNA, may be helpful in running calculations inside human cells to release drugs, or prime the immune system, at the first hint of illness. They have revealed that the RNA device processes input signals in the form of natural cell proteins and produces an output in the form of green fluorescent protein (GFP). At the computers heart is a ribozyme, a short RNA molecule able to catalyse changes to other molecules, which is attached to an RNA sequence that the cell can translate into GFP, and a third RNA molecule that acts like a trigger for the ribozyme. The team say that the trigger can be designed to bind to specific molecules inside the cell like proteins or antibiotics. When it does, the catalytic ribozyme destroys the GFP sequence, and prevents the cell from making any more glowing protein.
The presence of an input protein stops the production of GFP. Using two trigger sections produces a NAND gate, the output of which depends on the presence or absence of two input proteins. Smolke says that multiple NAND gates can be used to write any other logical operation, making them “the most celebrated example of a device enabling universal computation.” The team believe that it should be simple to transfer their computer into mammalian or bacterial cells in future, and to chain logic gates together to perform more complex operations. Any such success may offer a revolutionary approach to studying and healing biological systems, say the researchers. According to them, using strings of logic gates, a similar system could combine signals from several different biomarkers to produce more complex responses, for example, cocktails of drugs or hormones. “This is an important step forward in the field of synthetic biology and in vivo computing,” New Scientist magazine quoted Kobi Benenson at Harvard University as saying.
http://www.thaindian.com/newsportal/india-news/living-computers-within-body-may-revolutionise-biological-healing_100108283.html
See also
- AND gate
- OR gate
- NOT gate
- NOR gate
- XOR gate
- Boolean algebra
- Logic gates
