Timing of single point aquisition

[DavidT]

Hello everyone,

I would like to perform single point AI daq for a pid loop. I would also like to log the AI data to file with a timestamp. If i use 'single point hardware timed' daq, can i confidently attach a timestamp to each point which is determined by t0 and dt? I ask this because im running my loop on windows xp, and determinism of the OS is shocking (loop times jitter b/w 5 and 15ms). The pid determinism isnt that critical, but the logged data timestamp is.

Also, is 'continous aquistion' hardware timed or is this determined by the OS clock?

thanks

[Tom_W_[DE]]

Hi David-

The hardware-timed single point time t0 data comes directly from the system clock.  So while the timing itself is done in hardware the actual timestamp is only as accurate as calls into the O/S are time-wise.  Since it seems like you are having somewhat unreliable CPU timing issues this method might not meet your needs.

Finite and Continuous operations performed via the method shown in examples "Acq&Graph Voltage-Int Clk.vi" and "Cont Acq&Graph Voltage-Int Clk.vi" (Help>>Find Examples in LabVIEW and then browse to Hardware Input and Output>>DAQmx>>Analog Measurements>>Voltage) are completely hardware-timed and will be free of issues with O/S latency.  In other words, if you can accurately pinpoint the t0 of the first sample (again this might be difficult due to the fact that Windows XP is by no means a deterministic O/S) then you can be confident that time data for any subsequent samples will be absolutely correct.

Hopefully this helps-