Fast acquisition rates cause delay when viewing data on a graph

[OKors]

I'm using a cDAQ 9172 and 9215 AI module connected to a laptop running Signal Express, Core2 intel, Win 7, 4Gb Ram.

For my testing needs I rely heavily on being able to watch the data in realtime on the graphical display (chart graph, or xy).

My problem is, If I sample high enough to avoid aliasing (100Hz/10samples to read) the plot becomes heavily delayed by up to a few seconds. I'll put an input into the test rig and the graph will display that change in signal several seconds later. This is unusable for me. The only way I can view the data on the graph in realtime is to reduce my sample rate but then I'm left with horrible resolution.

Is this a throughput issue with the USB or timing related? I've tried various timing settings and nothing seems to land me in that sweet spot where I can view the data on the graph in realtime while maintaining higher sample rates.  

[OKors]

I should also add... This signal delay in the graph does not happen when the cDAQ is connected via USB to a PC with 4GB ram and equivalent CPU

[Emily-C]

Hi OKors,

Can you please give me more details on how your data collection and graph display are set up? How much data are you displaying at once, and what kind of graph are you using?

I understand that you want to see the results in realtime, but how much of a delay would be acceptable? If moving to a lower frequency would be acceptable to you as long as there wasn't aliasing, how low a frequency for printing new samples would be acceptable?

It sounds like your laptop is having trouble printing samples that fast. It could help if you used Defer Panel Updates. If it was acceptable to only update your graph 5 times per second, for example, you could create a loop that would collect data for 200ms before drawing the new data to the front panel. This way, you can still take as many samples as you want, but reduce the number of calls to the front panel, which could help with your delays.

This example shows how to get the property nodes set up: https://decibel.ni.com/content/docs/DOC-4519

I hope this helps!