Hello Matt,
Thank you for contacting National Instruments.
Try using the DIO Config.vi located under NI Measurements >> Data Acquisition >> Digital I/O >> DIO Config.vi. In the "handshaking mode parameters" cluster, set "edge mode" to trailing. I believe this will set your task to read on the falling edge of PCLK.
Let me know if you have any further questions.
Sincerely,
Sean C.
Applications Engineer
National Instruments