In ladder logic, can a rung be true if the coil is not energized, and why?

• A. Yes; a rung can be true independent of the coil state; energizing the coil depends on the rung conditions.
• B. No; a rung true requires energized coil.
• C. Yes; but only in online mode.
• D. No; coils and rung truth are unrelated.

Answer: (A)

In ladder logic, the truth of a rung comes from the input conditions (the series/parallel contacts) on that rung. The coil is just the output device that is energized as a result of that rung being true. So the rung’s logic can be satisfied based on the inputs even if the coil’s current state isn’t energized at that exact moment—the coil energization is the consequence of the rung evaluating to true, not what defines the rung’s truth. This is why a rung can be considered true independently of whether the coil is energized, because the coil state reflects the action taken after the rung is evaluated.