Digital measurement

This should be simple, but I can not seem to figure it out.
I am using a signal generator to generate a 1KHz 5Vpp (0 - 5V) signal and send it to an NI USB-6008 digital input. I want to log and view this signal.
I use DAQmx Acquire, Continuous samples, 10K samples to read, 10K sample rate. I assume this should give me a signal that looks somewhat like 10 samples low and then 10 samples high, and that the buffer should be refreshed every second.
What I get are samples that range from three samples high and three samples low to one sample high and one sample low and they are not consistent. At the very least I would expect a 50% duty cycle (as that is what I am putting in). By sampling at 10K (10 x the sampled signal) I thought there should be no problems... I even tried sampling at 100k but the results are much the same.
When I move the signal to an analog channel, it looks fine or if I tie an analog channel and a digital channel together, the analog channel looks as I would expect.
As I am new to this program, I expect I am just setting something up incorrectly, but I am having a hard time figuring out what. Any help would be greatly appreciated.
I have attached a text file of the output for reference.
Kind Regards,
Jeff
Attachments:
Digital Input - Dev1_port0_line1.txt ‏30 KB

Please find attached a screenshot of my desktop. The input is a 5Vpp 50% duty cycle 1 KHz square wave. It is connected to Digital Port 0 line 1 and Analog input line 0.
The top waveform is the analog input sampled at 5 KHz with 5k samples to read. As you can see each cycle is 1 mSec which is what I would expect.
The second waveform is the digital input sampled at 5 Khz with 5k samples to read. Here you can see the inconsistency in the waveforms. I would expect these to be 5 samples low and then 5 samples high but that is not what I am getting and I can not figure out why.
The bottom waveform is simply a power spectrum derived from the analog channel to see if there are any harmonics, etc.
Thanks,
Jeff
Attachments:
1Khz 50Pct 5Vpp.doc ‏2216 KB

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Get with your fellow hardware users :: [ NI's VirtualBench User Group ]
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Message Edited by Jaime F on 11-05-2007 12:57 PM
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Attachments:
Read Dig Port1.vi ‏31 KB
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Digital measurement

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