The OR Gate

Definition. An OR gate is a logic circuit with two (or more) inputs and a single output, whose output is HIGH ($1$) whenever ANY ONE, OR MORE, of its inputs is HIGH -- the electronic realisation of the everyday INCLUSIVE sense of the word 'or' (as in 'you may have tea or coffee or both'), and its output is LOW ($0$) only in the one case where every single input is LOW.

Symbol. The standard shield-shaped OR-gate symbol (drawn in the accompanying figure) has a curved, concave back edge where the input lines enter and a single rounded, pointed tip at the front where the output line emerges -- distinguishing it visually from the flat-backed AND-gate symbol of Section 9.27.

Truth table and Boolean equation. For two inputs $A$ and $B$, the OR gate's behaviour is completely specified by its truth table (shown alongside): output $Y=0$ only when $A=0$ AND $B=0$ simultaneously, and $Y=1$ in every other one of the four possible input combinations. Its Boolean (logic) equation is written

where the '$+$' symbol here denotes the logical OR operation specifically, and must never be read or manipulated as ordinary arithmetic addition -- notably, $1+1=1$ in Boolean logic (HIGH or HIGH is still just HIGH), not $2$, since a digital signal recognises only the two levels $0$ and $1$ and nothing in between or beyond. …