The link protocol is simple. There are 5 signals,
XREQUEST, XREADY, XENABLE, XVALID and XCLOCK.
When the XREQUEST, XREADY, XENABLE and XVALID are asserted,
the data should be strobed by XCLOCK on receiving and sendig
data. The detail of the protocol is in section 
.
XOCLK is a clock generated by P-PCI-LV. FADC system uses 20MHz clock
as XCLOCK for sending data.