Combining two signals so that the output is on if either signal is present. This can be accomplished by an OR logic gate (two inputs, one output which is high if either input is).
It can also be done with a "wired-OR" connection in which two signals are simply wired together and either one of them can raise the level. This works when the signals are driven by a source that only pulls up or only pulls down, with a resistive load (e.g. an "open collector" output).
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