5.2.3 CDR Far-End Loopback

CDR FELB (CDR far-end loop back), occurs after the parallel word creation in the PCS through a 
loop-back FIFO. The transmit word is then serialized and sent out of the transmitter.

This loopback is after the CDR which requires the receive and transmit paths to have exactly matched clock rates or 0 ppm differences. In the Far-end loopback case, the LANE#_RX_READY must be used instead of LANE#_RX_VAL to indicate valid data path.