digital output with a reference trigger

[MidhatT]

Dear all,

I am trying to generate a digital output based on an external sampling clock. The high and low times (or the initial delay and high time) is dependent on the pulses from the external clock. This external clock is a shaft encoder which sends out a finite number of trigger pulses (1800) every rotation. 

Now my issue is that I also need to trigger when to start the digital generation. This is also based on a pulse from an external clock. This pulse is generated once every rotation of the above mentioned encoder. For this I am using the Digital Edge start trigger vi. However I need to pretrigger my samples by a certain amount. This option to pretrigger is only available when I use the Digital Edge reference trigger vi. However when I use the reference trigger instead of the start trigger I get an error saying 'Property not applicable to this task or supported by the device'. 

I am using a PCIe-6351 DAQmx and this is a continuous generation thus inside a while loop. I do the same thing (use and external sample clock and external reference trigger) to collect analog inputs but do not face the same problem. 

Could some one please help  in this regard?

Thanks

[David. B]

Hi Midhat,

Thank you for posting this question in the forums.

There is a reason for the behaviour that you are seeing, the 6351 does not support Reference Triggering on Digital Output. The reason that you can see it on the Analog Input is due it supporting Reference Triggering on both the Digital and Analog Input but not on either of the outputs. Details on this can be found in the manual here under External Digital Triggers:

http://sine.ni.com/ds/app/doc/p/id/ds-100/lang/en

I'm a little confused why it is that you need to pretrigger an output? I hope that you can see in the manual why it is you are experiencing these issues and I shall try and help if you have more.

Kind Regards

[MidhatT]

Hey David,

Thanks for your reply. Just a quick question, are there any other PCI or PCI express DAQ cards that support digital ouptut reference triggering (for example the PCI 6254)?

The reason I need to reference trigger is, the shaft encoder is on an engine flywheel. Now the shaft encoder sends out a pulse that is supposed to indicate the top dead center position (the start of the engine cycle). However due to inaccuracies when fixing the encoder on the flywheel, the actual start of the cycle does not match with the pulse output from the encoder, hence the need to pretrigger the data acqisition or generation. 

I suppose I can mathematically compensate for that difference when generating the digital output, unless you can suggest a better solution?

Regards

Midhat

[David. B]

Hi Midhat,

I have not seen any and I know that the 6254 does not. I see why it is that you do need it, would you be interested in me getting one of our engineers in our sales department to contact you to see if there are any cards which fit your requirements?

Kind Regards