An I/O standard with a voltage-referenced signal, input (V_REF) of 0.9 V, an output (V_CCIO) voltage of 1.8 V, a GTL+/SSTL input buffer and a 1.8 V TTL output buffer. The SSTL-18 Class I & II I/O standard is supported by the Stratix^™ and Stratix^™ GX device families.