Read data from buffer relative to start of buffer

[crcarlin]

Idea:

We've come across a few use cases where it would be nice to pull samples from the DAQ buffer based on position in the buffer instead of sample number. This gets a little hard to describe, but a NI applications engineer referred to it as absolute addressing without regard to FIFO order.

In its simplest form we could use a read operation that just pulls from the beginning of the buffer as it is (probably?) in memory, maybe using a RelativeTo option of "Start of Buffer", with no offset.

The thought is that sometimes a properly set up buffer can contain exactly the data we need, so it'd be nice and clean to just get a snapshot of the buffer.

Use cases:

Our use cases involve continuously and cyclically sending AO and sampling AI in tasks that share a time base, ensuring that every n samples will be the beginning of a new cycle. A buffer of that same size n will therefore be circularly updated like a waveform chart in sweep mode.

In other words, the sample at the first position in the buffer will always be the beginning of the cycle, no matter how many times the sampling has updated the buffer.

If we could take a snapshot of the buffer at any moment, we'd have the latest readings made at every point in the cycle.

Alternatives:

The idea is that the buffer at all times has exactly the samples we need in the form we need them. What's lacking in existing functionality?

With RelativeTo First Sample, we don't know exactly what samples are there at any moment. We can query Total Samples and do math to figure out what samples the buffer contained, but while we're doing math sampling continues, leading to the chance that our calculation will be stale by the time we finish and we'll get a read error.

RelativeTo Most Recent Sample can return an entire cycle worth of samples, but they'll probably be out of phase. The sample beginning the cycle is likely to be somewhere in the middle of the FIFO order.

RelativeTo Read Position requires that we constantly read the buffer, which is a hassle if we only want to observe a cycle occasionally. It kind of means we'd be duplicating the buffer, too.

Best alternative:

In talking with engineers and on the forums, it sounds like the best option for us is to use RelativeTo First Sample and Total Samples to calculate the sample number at the beginning of a sample, and then make sure the buffer is many cycles long to mostly guarantee that the sample will still be there by the time we request it.

Forum post: http://forums.ni.com/t5/forums/v3_1/forumtopicpage/board-id/250/page/1/thread-id/91133

NI support Reference# 2407745