When you receive the input signal, the sequence will always execute on the next frame edge.
However, it does depend on how often the poll takes place and when the input was received with respect to the frame. If you put in 1 frame, the PLC is polled at least once per frame. But, if we "just missed" the PLC poll, then we will wait until one more frame edge before executing the sequence.
From the picture below, you can see that the discrete is actually more accurate.
