sorry. The 595 is one of several 74xxx595 "standard logic" chips. You should be able to find a datasheet by searching for "74hc595." I believe that the "shiftout" function mentioned in various places here is designed to write to 595-like chips, and will probably work with your UCN5821 as well. A similar scheme is used as the basis ("physical layer" if you will) for somewhat more complex protocols like "spi" and "i2c", so it's both common and quite general purpose.