An I/O standard used for high-speed I/O interfaces, the Differential HSTL I/O standard uses differential inputs without a reference voltage.

The Differential HSTL I/O standard uses a differential signal, with an output (V_CCIO) voltage of 1.5 V, as well as a termination (V_TT) voltage of .75 V, to which termination resistors are connected. The Differential HSTL I/O standard is supported by the APEX^™ II, Stratix^™, and Stratix^™ GX device families.