usb 6008 daq minus time value

[wyjohnny]

Hello,

I have a question about using usb 6008 daq board. I use that for voltage signal data acquisition, sampling at 4kHz and read at 1kHz. In addition, I have a usb 6210 acquiring another voltage singal at the same speed. They are connected to the same usb hub which then goes to the usb socket of a windows computer.

In the labview 8.2 program, 2 daq assistant were used in the same loop (no waiting time) and the two singal waveforms  goes to the same lvm file. 

The problem is that quite frequently I have minus time value in my lvm file. If this happens, you can notice from the chart for the signal that it stops updating. After a several seconds, it may go back to normal. But if you then look at the lvm file, you will see minus time values that corresponds to the period when the chart stops updating.

Thanks a lot, 

[Mark_SAI]

Hello,

Could you post your VI for me to look at?

Thanks,

[wyjohnny]

Thanks for reply.

Please find attached the vi and some example data file which contain minus time value.

The velocity and current.vi is the main vi.

Strangely, the minus time problems occur much less frequently if I use another computer.

Thanks

[Mark_SAI]

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Strangely, the minus time problems occur much less frequently if I use another computer.

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 This makes me think it could possibly be a USB bandwidth issue or something similar. Have you tried not using a USB hub, and instead plug both USB devices to different ports on the PC?

[Mark_SAI]

Hello,

I have looked at your code and noticed you use local variables- this will lead to race conditions and the loss of data, may I suggest using wires instead of local variables?

I hope this helps!