A system for providing a CMOS I/O circuit design that may replace existing bipolar I/O circuitry, and thus behave in substantially the same manner as bipolar I/O circuitry. Thus, an I/O circuit using a standard CMOS process is made that mimics operation of an ECL I/O circuit created using bipolar transistors. The CMOS input circuitry can receive input signals from an ECL output circuit, so as to mimic traditional ECL input circuitry. The CMOS output circuitry can output signals to an ECL input circuit, so as to mimic traditional ECL output circuitry. The CMOS I/O circuitry is designed to mimic the temperature dependent signals level, as present within traditional ECL I/O circuitry.