In the Quartus® II software versions 7.1 and 7.2, you cannot assign pin locations with differential I/O primitives. You may see this error during compilation of your design when you assign pin locations for top level pins in the HDL code using the following parameters in differential primitives:
input diffin_p, diffin_n;
ALT_INBUF_DIFF DIFF_IN (
.i ( diffin_p ),
.ibar ( diffin_n ),
.o ( tmp )
defparam DIFF_IN.io_standard = "LVDS";
defparam DIFF_IN.location = "PIN_AG18";
When compiling designs in the Quartus II software versions 7.1 and 7.2, you can use the differential primitives in HDL code for Stratix® III and Cyclone® III designs, but you must use Quartus II software Assignment Editor to assign pin locations for assigning the top level differential pins to device pin locations.
For more information on Assignment Editor, refer to Assignment Editor chapter in Quartus II handbook at http://www.altera.com/literature/hb/qts/qts_qii52001.pdf.
For more information on differential I/O primitives, refer to Quartus II help.
This issue is scheduled to be fixed in a future release of the Quartus II software.