Square wave irregular by 1-2 ms on oscilloscope

[UselessIntern]

using an NI-pxi-1031 box, with a pxi-6281 card.  Outputting square wave function (digitally).  The wave comes out fine, but at frequencies approaching 1kHz the wave begins to jump around on the oscilloscope, the period shifts by a millisecond or two.  Is this a hardware limitation, or setting?  Is there any correction for this?

[Christian Mergl]

Hey,

What exactly do you mean by "...begins to jump around on the oscilloscope, the period shifts by a millisecond or two..."

Are you using AO or correlated DIO?

Christian

[Ben]

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UselessIntern wrote:
using an NI-pxi-1031 box, with a pxi-6281 card.  Outputting square wave function (digitally).  The wave comes out fine, but at frequencies approaching 1kHz the wave begins to jump around on the oscilloscope, the period shifts by a millisecond or two.  Is this a hardware limitation, or setting?  Is there any correction for this?

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That is the natuer of Windows. Its fine for tricking people into think it is opperating real-time provided real-time is less than 30 Hz like in a screen update or a mouse move. At 1KHz Windows performs as you have observed. There are two things you can do under Windows that may help. If you hardware is capable of buffered digital output (hardware will clock data not Windows) or to use a "Timed Loop" which will perform better but is not bullet proof.

Ben