General questions regarding 6501 output controls

[heepofajeep]

Hi, I am using labview 8.0 to control a stepper motor through a stepper motor driver that receives ttl logic in terms of step & direction.  The problem I have right now is the outputs are not timed very well.  I suspect this is a coding issue, but I was wondering what the highest reliable frequency that I can expect out of labview & the NI usb-6501 is.  Also, where is there any where I can view examples of output control with the 6501?  Thanks!

[Justin_M]

Hi heepofajeep,

How fast will you need to change the logic values?  The 6501 uses all software timing, meaning it can only respond as fast as your OS can send out the commands.  Take a look at the following benchmarks, which should give you a good idea of the rates you can expect to see:

USB-6501 Benchmarks with NI-DAQmx

I hope this helps!

Thanks,

Justin M.
National Instruments

[Jonathan_Brumley]

I would like to point out that the combination of software timing, USB, and Windows (or Linux) is particularly bad for a control application that needs guaranteed timing and good responsiveness.  The benchmarks Justin pointed you to are reporting 'average" update rates.  On any desktop OS like USB and Windows, there can be lots of jitter inherent in the operating system, drivers, and services that may be running.  The OS can switch to another task at any time, which messes up any guarantee of timeliness.  So you can expect updates to occasionally take much longer than average.

If you need a more reliable or faster solution, I would recommend different hardware and software.  A DAQmx write to a PCI or PXI device is typically 50 times as fast as the same write over the USB bus.  In addition, the LabVIEW ETS real-time operating system is much more deterministic than any desktop OS.