OR gate
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- This article is about OR in the sense of an electronic logic gate (e.g. CMOS 4071). For OR in the purely logical sense, see Logical disjunction, for other uses of similar terms, see OR.
| INPUT A B |
OUTPUT A OR B |
|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
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.
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[edit] Symbols
There are two symbols for OR gates: the 'military' symbol and the 'rectangular' symbol. For more information see Logic Gate Symbols
[edit] Hardware Description and Pinout
OR Gates are basic logic gates, and as such they are recognised in TTL and CMOS ICs. The standard, 4000 series, CMOS IC is the 4071, which includes four independent, two-input, OR gates. The pinout diagram is as follows:
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1 Input A1 2 Input B1 3 Output Q1 4 Output Q2 5 Input B2 6 Input A2 7 VSS 8 Input A3 9 Input B3 10 Output Q3 11 Output Q4 12 Input B4 13 Input A4 14 VDD |
This device is available from most semiconductor manufacturers such as Philips. It is usually available in both through-hole DIL and SOIC format. Datasheets are readily available in most Datasheet Databases.
As well as the standard 2-Input OR Gate, 3- and 4-Input OR Gates are also available:
- 4075: Triple 3-Input OR Gate
- 4072: Dual 4-Input OR Gate
[edit] Implementations
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[edit] Alternatives
If no specific OR gates are available, one can be made from NAND gates in the configuration shown below. Interestingly, any logic gate can be made from a combination of NAND gates
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.
