daqmx independent triggers

[miliescu]

Hi, I'm using an S series PCI 6115 at high AI sampling rate (10 MS/s), in conjunction with other NI devices. The acquisition is set in pretriggered mode, so I use a digital edge reference trigger. I need to independently control each channel trigger. It could be done using analog triggering mode, but this is not appropriate for my application, where I have to measure also baselines, which do not give analog trigger. The precise timing is fundamental and this request excludes analog triggering, as well. I don't know how to obtain 4 independent trigger marks (digital edge reference), one for each channel, in order to extract the right data sets(pre/post trigger), for uncorrelated inputs.

In short, I want to use my 6115 as a set of 4 independent ADCs (as presented by the NI S series description), each channel with its own gain and (digital) trigger. The limitation to a common clock for all 4 inputs (due to the pipelined ADC) is fine to me. For compatibility between different boards I'm using DAQmx (cannot use traditional NIdaq, unfortunately..) with LabView, as first approach, but a C code solution would be fine, too. Thanks for any suggestion,

Mihai

[Hudson]

Now from what you described, it sounds like you are wanting to use a separate reference trigger for each individual AI channel because of the timing differences between your channels. This functionality is not possible even with your board as you can not set multiple tasks for the same operation (i.e. multiple Analog Input tasks).

You can specify one reference trigger for a group of channels but not a separate trigger for each channel. You have to keep all of the analog input channels in one DAQmx task and they will all share the same reference trigger. When you configure an analog input task, the analog input circuitry in your card is reserved for that task and hence you would not be able to configure any other analog input task regardless of the type of measurement it is because already the Clocking and other circuitries are reserved for the first task.

IF you want to configure your device for digital edge reference triggering to independently control each channel, one thing you might consider is performing post analysis to just keep the data you need. With the maximum sample rate of the PCI 6115, you shouldn't have too much of a problem keeping up with the acquisition. This isn't the most convenient solution, but it's definitely possible. To do this just start acquiring on all of the channels at the same time (This is basically what the card is doing when you set up a reference trigger, the data from the cards buffer is not returned until a trigger is received).  Once you are done with your acquisition you can look at the individual waveforms and keep just the data that is above a certain threshold.  This should yield the same results as reference triggers for each channel but you will be doing it after the acquisition has ended instead.

[miliescu · Accepted solution]

Thanks for the info, meanwhile I've found a solution quite similar to what you suggested: instead of independent triggers, I've send the OR of the 4 triggers to the unique edge reference available and, in parallel, the map of the channels I'm interested on, to the digital ports. Luckily, 6115 can synchronize digital and analog inputs; then, from the synchronized buffers I extract the channel data for each trigger line (using 4 out of the 8 available DIO one can process also triggers arriving on all channels at a time). Again, thanks for your detailed reply.

Mihai