Multiple DO channels with different frequencies

[DannyN]

Hi,

I am using NI USB-6221 card.

Using the C API I need to generate 6 Digital Output channels, with diffrenet frequencies and duty cycles.

To be more precise, 2 are totaly identical, but I need them to be inverted, and the other 4 are similar to one another, but should be shifted in time (I.e. there is a delay between each of the channels).

I used the 2 CO channels that the USB-6221 supports for the first two signals, and it works fine (the two signals are synchronized, and are inverted).

Now I need an additional 4 DO channels for the other square waves.

I saw an articale in the NI web site by JohnP with the title: 

Generate Multiple Channels of Digital Output with Different Frequencies and Duty Cycles

The following example shows how one might build and generate a digital waveform using non-regeneration so that you can update frequency and duty cycle on-the-fly with M or X series DAQ devices.  The example uses digital output rather than counters to achieve this, so if you need more outputs than counters available this would be a good option (note:  on M Series devices an external sample clock must be provided, this can come from one of the counters if desired).

which seems to be exactly what I need, but the examples are for LabView which I don't have.

Can someone explain how to do this with the C functions?

Best Regards,

Danny.

[John_P1 · Accepted solution]

Hi Danny,

The important thing to note is that you can clock out arbitrary digital waveforms (up to 1 MHz on the 6221).  The actual DAQ programming is quite easy once you have the waveform.  My LV Example used LabVIEW's Basic Function Generator VI, then converted it to a Digital Waveform to build the signal for each channel.

The LV functions helped immensely with enabling the waveforms to be updated on-the-fly (the Basic Function Generator keeps track of phase for you). 

If you don't need to update on-the-fly, then building the waveform in C shouldn't be too bad.  For example:

    p0.0        [1   1   1   1   1   1   1   1   1   1   0   0   0   0   0   0   0   0   0   0]     *1

    p0.1        [0   0   0   0   0   1   1   1   1   1   1   1   1   1   1   0   0   0   0   0]     *2

    p0.2        [0   0   0   0   0   0   0   0   0   0   1   1   1   1   1   1   1   1   1   1]     *4

    p0.3        [1   1   1   1   1   0   0   0   0   0   0   0   0   0   0   1   1   1   1   1]     *8

                    [9    9   9   9   9   3   3   3   3   3   6   6   6   6   6   12   12   12   12   12]

The above array of U8 would give you 4 outputs of 50% duty cycle waveforms at Fs/20, shifted 90 degrees from eachother.  The lines would be p0.0 through p0.3 (the binary bit of the U8 corresponds to which line goes high).

Best Regards,