time to pass data to half buffer in a double buffer acquisition

[Gonzalo]

Hello all:

I'm building a DAQ system with CVI 6.0 and an old Lab-pc+. I don't have access to the hardware at the moment and I was thinking how much time it will take to DaqDBTransfer function to pass 75750 points to the halfbuffer. Someone who know this?

Other question for the group:

Imagine that I'm programming this for acquiring with Lab_ISCAN_Start and timebase 1us with sample interval 33 which means 30303,030303 Hz (3 channels, or 10101,01010 Hz/chnl). I need to know if it's certain to say that point n acquired has happened exactly at n*3.3*10E-5 seconds. which is, for example, that point 80000 has been acquired at 2,64 s. from the beginning of the Scan. And how accurate is that.

thank y
ou all in advance.

[Russell]

Hello Gonzalo,

Be aware that this depends on the number of channels in your scan list and where it is in the scan list. In the Lab_ISCAN_Start() function there are two numbers you are concerned with. The sampInterval and the sampTimbase. (And the scanInterval if you choose to use this.)

First, determine when the particular channel you are interested in is sampled. This depends on which channel it is in the scan list. The Nth channel will be sampled at N*(sampInterval*sampTimebase) seconds.

Then, your scan period. The scan period will either be the scanInterval, or (if scanInterval is 0, 1 / (number of channels * (sampTimebase * sampInterval)).

The Mth sample of channel N will be sampled at

M*(number of channels * (sampTimebase * sampInterval))
+ N*(sampInterval*sampTimebase)

You might also look at the NI-DAQ Function Reference Help for this function for more information. This is accurate to within the base clock specification in your boards user manual.

Also, how long will it take to pass 75750 points with DaqDBTransfer? You can get a good estimate by using the above formula. Then you can guess there will be a few us of software overhead for the actual transfer.

Also, here are a few links you might find helpful.

Lab-PC+ User Manual
http://digital.ni.com/manuals.nsf/webAdvsearch/53215E9F6E1D0EA78625665E006359B4?OpenDocument&vid=niwc&node=132100_US

NI-DAQ Function Reference Manual for PC Compatibles
http://digital.ni.com/manuals.nsf/webAdvsearch/1F23F2C8FB44D660862566960071D243?
OpenDocument&vid=niwc&node=132100_US

and this is a good one...

Data Acquisition Definitions: Scan Rate, Channel Rate, Sampling Rate, Scan List, Interchannel Delay
http://digital.ni.com/public.nsf/websearch/DBE7AC32661BCF9B86
256AC000682154?OpenDocument

Anyway, I hope this helps!

Russell
Applications Engineer
National Instruments
http://www.ni.com/support