Programmable Logic Controller (PLC) Practice Test

An OR gate behaves like a parallel circuit. In a parallel setup, the output can be energized if any one of the input paths is closed, so just one input being true is enough to turn on the output. That matches the OR operation: the output goes high if any input is high.

If the inputs were wired in series, you’d need both inputs to be true to complete the path, which is the behavior of an AND gate, not OR. A bridge circuit describes a more complex network that isn’t the simple one-output OR behavior. An open circuit has no complete path at all, so the output can’t be energized regardless of input.