Hi,
Jeff and I are looking at this… and we now understand the problem.
This is a multiplexed device. It uses one A/D to successively acquire the defined channel list.
When you acquire one channel, the number of samples and the counter input match. It takes 1000 ticks to acquire 1000 samples.
When you acquire two channels, it takes TWO clock ticks to acquire them, so the number of samples per channel is 1/2 of the counter input, or, the counter input is double the number of samples per channel. It takes 2000 ticks to acquire 1000 samples for each channel, a total of 2000 samples.
When you acquire three channels, it takes THREE clock ticks to acquire them, so the number of samples per channel is 1/3 of the counter input. It takes 3000 ticks to acquire 1000 samples per channels, a total of 3000 samples.
Here's how I got it to work:
External timer set to 3,000 hz.
Number of channels = 3
DASYLab MCC-DRV Analog Input timebase, HW, set to 1000 samples/channel, block size 32
DASYLab MCC-DRV Counter Input timebase, SW, set to 100 samples/channel, block size 1
I also changed the Relay setup to more accurately stop after 5 seconds. The Action module is always slightly delayed (asynchronous, typically), so I try not to use it.
I also added a STOP module to stop the measurement after 5 seconds.
Another issue -- when you use external timing, the minimum block size is 32, and should be a multiple of 32 for higher sample rates. That's how this device works - it has a minimum buffer of 32. Use block sizes of 32, 64, 128, 320, 640, etc… try to keep the block size about 1/10 of the sample rate per channel.
Every time you change your input sample rate, configure the input timer so that it is a multiple of the number of channels that you are collecting, times the sample rate per channel. Configure the Analog Input timebase to match the sample rate per channel.
In my scenario, this is line 5007 of the file (the last line)
5 seconds, 157 blocks, 5001 samples per channel, and 15,017 clock ticks to do that.
5.000 157.00 5001.00 0.00 1.75 15017.00 1.52
I changed the external timer at 30,000 Hz, and the Analog Input timebase to 10,000 samples/second, block size 320.
This is line 50,007 of the data file.
5.0000 157.00 50001.00 0.00 1.75 150050.00 1.42
Because of the discrepancy of the sample rates for the Analog Input and the Counter input, those values are never going to match exactly.
Let me know if this helps.
Thanks,
-cj
Catherine Jo "cj" Butler | Account Manager and DASYLab Product Marketing
Measurement Computing/Data Translation
10 Commerce Way | Norton, MA 02766
Direct: 508-342-3042
