The Pin Connections Guidelines document for Stratix® IV GX/GT, Arria® II GX/GZ, and Cyclone® IV GX devices, recommends to connect unused GXB_Rx and REFCLK pins to GND through 10k resistors. The reason for this recommendation is to improve noise immunity. However for applications where the receiver or REFCLK is routed to an unused connector, the receiver would effectively be floating. In this instance it is OK to not tie the unused receiver to GND through a 10k resistor.
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