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A + B
The OR gate is a digital logic gate that implements logical disjunction - it behaves according to the truth table to the right. A HIGH output (1) results if one or both the inputs to the gate are HIGH (1). If neither input is HIGH, a LOW output (0) results. In another sense, the function of OR effectively finds the maximum between two binary digits, just as the complementary AND function finds the minimum.
There are three symbols for OR gates: the American (ANSI or 'military') symbol and the IEC ('European' or 'rectangular') symbol, as well as the deprecated DIN symbol. For more information see Logic Gate Symbols.
|MIL/ANSI Symbol||IEC Symbol||DIN Symbol|
OR Gates are basic logic gates, and as such they are available in TTL and CMOS ICs logic families. The standard 4000 series CMOS IC is the 4071, which includes four independent two-input OR gates. The traditional TTL version is the 7432. There are many offshoots of the original 7432 OR gate. All have the same pinout but different internal architecture, allowing them to operate in different voltage ranges and/or at higher speeds. In addition to the standard 2-Input OR Gate, 3- and 4-Input OR Gates are also available. In the CMOS series, these are:
TTL variations include:
module(a,b,c); input a,b; output c; or (c,a,b); end ;
If no specific OR gates are available, one can be made from NAND or NOR gates in the configuration shown in the image to the right of this text. Any logic gate can be made from a combination of NAND or NOR gates.
|NAND Construction||NOR construction|
With active low open-collector logic outputs, as used for control signals in many circuits, an OR function can be produced by wiring together several outputs. This arrangement is called a wired OR. This implementation of an OR function typically is also found in integrated circuits of N or P-type only transistor processes.
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