How to setup AI to AO

Question

I know this is probably possible, I just don't know how to
best go about doing it. I would like to write a program to sample a waveform
off of AI0 and output that same waveform on AO0 real time (or as close to real
time as I can get) - with the eventual goal of doing some processing on that
waveform in between. But for right now all I am trying to do is create a
program to do AI to AO.
The examples I have seen for doing simultaneous AI/AO have the output fixed
while it samples data on the input. That is not what I want. I want to be able
to spit out the signal that is present on the input ADC. Also when I try to
modify the program, I get these long (5+ second) delays in between what is
present on the input and what it produces on the output. I have tried
increasing the sample rate and/or reducing the number of samples but that just
causes errors – I’m loosing data because one process is taking too long to
complete.
Is there a way to make a streamlined program to simulate an
analog buffer?
Details:
NI Device: USB 6251
Software: Labview 8.5
Preferred Sample Rate: 40 kHz
Preferred delay AI to AO: < 0.1 second
I have no code started that I could post. I am wondering if
anybody has done this or has an idea about how to do it, i.e. I need to use a
queue to buffer data from the AI chain to the AO chain.
Thanks
Jim

Answer 1

Hello Jim,
Andre is correct that our main concern here is going to be the buffer
delay.  There are 2 other main delays that we need to be concerned
about as well.  These are  software processing and USB bus transfer
rates.  Andre's example for the output should work, but I believe it is
a little more complicated than you probably need.  He is using a state
machine architecture with an event structure, which is easy to expand
but can be difficult to follow for a task such as yours.
The producer/consumer would work as well, especially if the processing
you are planing on implementing is significant, but this too I think
may be beyond your initial goal.  This is, however, a great way to get
rid of "loosing data because one process is taking too long to
complete."
The reason you are seeing the delay is most likely due to the buffer
clearing out.  When you call the DAQmx write, it puts the data at the
end of the buffer, so all the data ahead of it needs to be output
first.  You can reduce this delay by using non regeneration (see the
example program named Cont Gen Voltage Wfm-Int Clk-Non  Regeneration.vi
in the Example Finder) and writing smaller chunks (samples to write). 
The trick with this is you will get an error if you do not keep some
data in the buffer that has not been written.
You also might want to get rid of buffers all together and use hardware
single point timing for your output (instead of continuous).  This is
when you write 1 point at a time to the card and it is output on the
next hardware timed sample clock pulse.  You can use this in
conjunction with the Wait for Next Sample Clock.vi (in the DAQmx Real
TIme palette).  The problem with this is that because you dont have a
buffer, you can only go as fast as you can write points (my computer
could go about 500Hz if I did not do anything else at the same time).
The USB is a slower bus than, for example, PCI.  This means that it takes longer to get data to and from the board (there is a lot of overhead).  This could become an issue at the rates you want.
I hope this gives you some ideas.  Let me know if you want me to go into more depth about anything or if you have any questions.
Message Edited by Neal M on 06-17-2008 01:20 PM
Neal M.Applications Engineering       National Instruments        www.ni.com/support