This answer record offers some basic debugging tips for Loopback.
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The Xilinx GTs provide various loopback modes to test out the various paths of the transceiver on both TX and RX side. This can be used with either user data or with the internal PRBS generator and checkers in the GTs to validate the test path.
The loopback test modes fall into two broad categories - near-end loopback modes where the TX data is looped back closest to the traffic generator and the far-end loopback modes where the data is looped back at the far end of the link.
Refer to the Xilinx transceiver user guides (UG476/UG482 for 7 Series and UG576/UG578 for UltraScale and UltraScale+) "Loopback" section under Chapter 2 - Shared Features for complete details on the various loopback modes available.
When switching between loopback and normal modes please be aware that an RX reset (GTRXRESET) is required after entering and exiting near-end PMA loopback or far-end PMA loopback. The loopback modes are typically enabled through the LOOPBACK[2:0] port.
The following answer records provide some additional information regarding loopback modes:
|(Xilinx Answer 47328)|
|(Xilinx Answer 53107)|
|(Xilinx Answer 64062)|
|(Xilinx Answer 65719)|