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On the EZIO2X4, Inputs 1 and 2 are opto-isolated and there is an internal resistor that keeps the logic level “put” so noise is not an issue whenever the inputs are left disconnected. Because Inputs 3 and 4 can be used as analog inputs, there is no termination on them, meaning that their logic level can be indeterminate when the inputs are left disconnected. This causes the actual logic level seeing internally to “bounce” between logic levels causing the issue some of you have seen. The pull-up resistors simply take the inputs and force them to a given logic level.