Description

Virtex-II devices have LVDS current-mode drivers that eliminate the need for source termination in point-to-point applications. Why is it that these drivers cannot be used for BUS LVDS?

Solution

The current-mode driver has a fixed impedance of 50 ohms, and it has no receiver termination. If this were used for a bidirectional I/O, the receiver termination would be affected by the fixed 50-ohm value; even worse, you would have to put an external 100-ohm resistor (or an approximation based on your bus size) in parallel to ensure correct receiver termination.  

Thus, you would be double-terminating the transmitter, which would completely ruin the DC characteristics of the driver.