Problems with single point acquisition on PDA with 6024E

[Crazydriver]

I have problems with single point acquisition using a pcmcia card 6024E on PDA with Labview 8.0 (but I have the same problem with Labview 7.1)

I created a little application to test continuous data acquisition.

My application includes a chart, a numeric indicator that display the time (ms) between two cicles and a boolean control that turn on and off the chart (using a case structure)

With this configuration I obtain a real time display of the input signal (I use a sine wave at 1Hz) but it's a false real time! If I stop the input signal, the chart still display the signal for some seconds!

With chart off the difference between two cicles is about 20ms (50Hz!), but when I turn on the chart the difference is about 17ms and the signal is not in real time but it appears "buffered".

If I increase  the scan rate or the number of scans, the delay grows.

DAQmx base configuration utility settings are:

- input channel: ch0

- scan rate: 50 Hz

- number of scans: 1 (or 0)

( for labview 8.0 users: timing mode finite on continuous)

I need to acquire with my PDA a continuous signal with a refresh rate of 5 or 10 Hz and a scan rate of about 150 Hz....and to visualize it using a chart!

Someone it has a solution?

I cited my VI

Thank you!

Marco

[Nick_]

Hi, I can reproduce your problem with my ppc and your VI.
I believe the lag we see is not introduced by the daq board (I created a channel in "on demand" timing mode and the problem still arise) but by the pda that is not fast enough to keep up with display update. (I believe those update callbacks are deferred...)
Examples that come with the nidaqmx base behave differently, they use a graph instead of a chart and each update show actual signal.
I obtained better results also with a chart fetching 10 or 20 points instead of 1, in this way your application refreshes indicators only one time each 20 samples.
Also consider that buffering your channels ("continuous" timing mode) helps you to obtain evenly sampled data and to be less dependent from PDA jitter, PPC is not a RT os, so it does not assure you to achieve a task in a specified and precise time.  Visualization of your buffered samples could be delayed by lack of resources, but the data you obtain would be somehow time stamped and you can keep track of when something happened.

Hope this help.

Ciao.

NicolaC