Use an AND gate, derived from two NOT ANDs, to create a circuit that blocks and passes the 20 kHz wave from the function generator. The NAND gate has ground on pin 7 and 5V on pin 14. The signal from the FGEN should be a square wave at a frequency of 20 kHz, with a low voltage of OV and a high voltage of 5V. The U2A and U2B comprise an AND gate, as the first is a NAND and the second is an inverter. When the DI/O line is high, the square wave from the function generator passes through. When the DI/O line is low, nothing should transmit.