External Triggering AI on DT9844

[Grata]

We are using an external trigger and an external clock to acquire a signal on AI0 of a DT9844.

The external clock is a square wave, 50% duty cycle, 500KHz

The external trigger is a square wave, 50% duty cycle, 300Hz.

The AI0 input is a a square wave, 50% duty cycle, 150Hz, 0 to 5V

AI0 is configured to scan the channel list (AI0 only) 100 times, to collect 100 samples per scan and to buffer a total of 10000 samples to display.

We anticipated the displayed buffer to have 10000 samples, looking like a square wave of 100 samples at 5V followed by 100 samples of 0V repeated 50 times.

 

We were able to configure an NI6343 to collect in this manner and the displayed output was what was expected.  The block diagram and output display from the NI6343 are show below.  The display shows the signal for one triggered scan and the complete buffer.

 

Also shown below are the block diagram for the DT9844 and the output display.  While the display for one triggered scan is what was expected, the complete buffer was not.

 

The number of samples per scan, scan count and buffer size may not be configured properly and are likely the cause for the 10000 buffered samples to seem to be random.

It there an improper configuration for these parameter in the block diagram? 

[Fausto]

Hello @Grata.

As you mentioned in your post, "one triggered scan is what was expected".  However, when retriggering the channel-gain list, there is a maximum retrigger frequency to consider.  Page 77 of the DT9844 User's Manual has that formula. 

https://www.mccdaq.com/PDFs/manuals/UM9844.pdf

 

 

You specified an external trigger rate of 300 Hz, but given your other numbers, your calculated maximum retrigger frequency should be ~50 Hz.

 

Regards,

Fausto

[Grata]

based on the configuration, minretriggerperiod should be:

 

(1 CGL * 100 CGLs per trigger)/500kHz + 2us = .000202s

This implies a max retrigger of 1/.000202s or 4950Hz.

the configuration calls for a retrigger at 300Hz which should be supported, correct?

 

If there is only 1 CGL (AI0) and samples per channel is set to 100 and scan count is 100, should there be only 100 samples collected per trigger?

Edited April 21, 2023 by Grata