For the Virtex-7 GTX IBERT core, there are two different modes that can be used which will effect how the design is generated.
Quad Based Protocol Selection
The first mode is called Quad Based Protocol Selection.This mode helps to conserve clocking resources in the fabric. When this mode is used, it assumes that the entire QUAD will be receiving data from the same source.This does not allow you to configure each transceiver in the QUAD individually, so all four transceivers in the QUAD will have the same settings.
GT Based Protocol Selection
The second mode is called GT based Protocol Selection.This mode uses more clocking resources, but allows you to configure each transceiver with individual settings.
When using the Quad Based Protocol Selection, the RXRECCLK from channel 3 of the quad (for example, GTX112_3) is used to drive the other three transceivers.This is generated in this way to save global clock resources. Because of this limitation, channel 3 within a QUAD must be connected on the board in order for the entire QUAD to work properly when using an asynchronous interface.
If channel 3 is left unconnected on the board when using this mode to generate the IBERT design in asynchronous operation, a high BER with the other transceivers will be observed since RXRECCLK is not available to the other channels. If channel 3 cannot be guaranteed to be connected on the board, it is recommended that the GT Based Protocol Selection mode be used to generate the IBERT design.