The TXBUFDIFFCTRL<n>[2:0] controls the strength of the pre-drivers and TXDIFFCTRL<n>[2:0] controls the transmitter differential output swing of Virtex-5 GTP transceivers.
The general recommendations for Virtex-5 GTP is to tie the TXBUFDIFFCTRL port to the same value as TXDIFFCTRL as stated in the Virtex-5 FPGA RocketIO GTP Transceiver User Guide (UG196):
The TXBUFDIFFCTRL and TXDIFFCTRL settings used for SATA protocol in the Virtex-5 FPGA SATA Generation 2 Protocol Characterization Test Report (RPT087) and the reference design in the SATA Physical Link Initialization with the GTP Transceiver of Virtex-5 LXT FPGAs application note (XAPP870) are as follows:
TXBUFDIFFCTRL<n>[2:0] = 3'b001
TXDIFFCTRL<n>[2:0] = 3'b100
The transmitter output swing settings to be used in an end user design are system dependent.
The general recommendations of tying TXBUFDIFFCTRL to TXDIFFCTRL as stated in the user guide or the settings referenced above in the SATA characterization report or XAPP870 reference design can be used as an initial starting point only.
Please note that the SATA protocol wrapper in the Virtex-5 FPGA RocketIO GTP Transceiver wizard follows the general recommendations.