The Logic OR Function |
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The Logic OR Function
The Logic OR Function function states that an output action will occur or become TRUE
if either one "OR" more events are TRUE. The OR function is sometimes called by its full name
of "Inclusive OR" in contrast to the "exclusive OR" function we will
look at later in tutorial 6.
The logic or Boolean expression given for a logic OR gate is that for
Logical Addition which is denoted by a plus sign, (+). Thus a 2-input
(A B) Logic OR Gate has an output term represented by the Boolean
expression of: A+B = Q.
Switch Representation of the OR Function
Here the two switches A and B are connected in
parallel and either Switch A OR Switch B can be closed in order to put the light on.
Again as with the AND function there are two switches, each with two
possible positions open or closed so therefore there will be 4 different ways of arranging the switches.
Truth Table
| Switch A |
Switch B |
Output |
Description |
| 0 | 0 | 0 | Both A and B are open, lamp OFF |
| 0 | 1 | 1 | A is open and B is closed, lamp ON |
| 1 | 0 | 1 | A is closed and B is open, lamp ON |
| 1 | 1 | 1 | A is closed and B is closed, lamp ON |
| Boolean Expression (A OR B) | A + B |
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OR Gates are available as standard i.c. packages such as the common TTL
74LS32 Quadruple 2-input Positive OR Gates. As with the previous AND Gate,
OR can also be "cascaded" together to produce circuits with more inputs such as in security
alarm systems (Zone A or Zone B or Zone C,etc).
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