An I/O standard with an external reference (V_REF) voltage of .75 V, 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 HSTL Class I & II I/O standard is supported by the APEX^™ 20KC, APEX 20KE, APEX^™ II, ARM^®-based Excalibur^™, Mercury^™, Stratix^™, and Stratix^™ GX device families. The HSTL Class III & IV I/O standard is supported by the Mercury device family only.