Take a look at the example program "Cont Gen Voltage Wfm-Int Clk-Variable Rate.vi"
This vi allows you to change the output frequency on the fly by adjusting the output sampling rate. Use your digital data to toggle between two desired frequencies (i.e. 10 kHz, 11 kHz). This would be like FSK (Frequency Shift Keying). You would need to set it up so that when no data is being sent the desired frequency could be a third default value (i.e. 9 kHz).
On the demodulation side, you would set up two bandpass filters at the two frequencies that represent your data (i.e. 10 kHz, 11 kHz). As you read data in, split it and run it through both in parallel. Take the absolute value of the outputs and lowpass filter them. A higher DC value in one of them would mean data is present and its digital value would depend on which filter produced the higher DC value. If both are have little or no DC value, there is no data.
I doubt you can get a very fast data rate, but this should work.
Randall Pursley
