NE PCS (LOOPBACK[2:0]="001")
The RXBUFFER cannot be bypassed.
The RXBUFFER is used to compensate for the phase mismatch between TX and RX side.
NE PMA (LOOPBACK[2:0]="010")
No known limitations.
FE PCS (LOOPBACK[2:0]="110")
TX and RX must be synchronous RXUSRCLK and TXUSRCLK must come from the same source.
FE PMA (LOOPBACK[2:0]="100")
The TXBUFFER cannot be bypassed.
The TXBUFFER is used to compensate for the phase mismatch between RX and TX side.