I had a similar issue with the USB-8451 until I realized that when initializing the port direction for the device, the value you give it is a bit mask and not just a number. I don't think you are having that issue, but it may come up later. Here is a VI that I've been working on for reading and sending to the DIO ports, the first part is setting up the device, might be what you need. Mine Is set up to write to P0.3 and P0.2, which are physically wired to P0.1 and P0.0 which are setup as inputs. I put this VI together to simulate reading a clocked digital signal. Hope it helps.
