Testers are synchronous devices, and need cycle level accuracy.
As shown in the conceptual model in Fig. 24, serial
links protocols for FBDIMM 1.0 and PCIe 1.0 use embedded
clock within data that are skewed independently and arbitrarily.
Therefore, there is indeterminism on the recovered clock-data
bundles with respect to received data; in addition, the recovered
byte clocks for the bundles are mesochronous. The transmit side
is similar and no better.