problem with digital output phase

[Tibo_94]

Hello, i am student and beginner in Labview, and i have to generate two signal with a phase of 120 degrees between them, by using digital outputs. i use a PCI-6229 card.

i put one signal on P0.3 and the other on P0.5, and when i have a frequency of more than 80 Hz, the phase is not 120 degrees. it's more or less, and far from the result i expected. and the duty cycle of the signals is not good too when i increase the frequency. i don't know where the problem come from.

to generate my signal, i use VI Simulate signal and i go to a DAQ Assistant for my output. i take acquisition with one sample on demand.

i am a little bit lost for that problem so if you can help me, that will be great.

Thank you.

Best regards.

Thibaud Duret

[johnsold]

Thibaud,

The clue is here: " i take acquisition with one sample on demand."

You are apparently using software timed output. To maintain accurate phasing at 80 Hz you need timing which is accurate to better than 4 milliseconds. This is not likely to occur with a desktop operating system. Sometimes it will be good and other times OS latencies will throw the timing way off.

If your board supports it, use hardware timing. The timebase sources on most boards are far more precise and accurate than software timing.

Lynn