Can U2651A work with 3.3V input signal or not?

As long as there is a potential exceeding 10V and less than 24V between DIin and DIcom, the digital input should be able to observe the incoming signal as logic high. This is a hardware behavior of the photo coupler that is being used in the U2600A unit. So, to answer your question if a 3.3V signal were used, the U2600A unit will not be able to process this signal as it is too small a signal for it to “see”.