enable differential mode on alt_iobuf??

I have a project based on and Arria 10 SoC development board which using FMC ports (LVDS IO Standard). I have a bidrectional pin which is the "io" port in an alt_iobuf instance.

The project synthesizes fully, but fails on the fitting, giving the error " can't on open-drain option for differential io pin".

I have looked up the solution and everywhere says to enable differential mode on the IO buffer, but nowhere says how to enable it?. Here is the current instance:

alt_iobuf IOBUFF_scl (

.i(1'b0),

.oe(scl_oen),

.io(scl), //bidirectional pin

.o(scl_c_normal)

);

How do you enable differential mode?

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