Scans...Buffering...Terminology?

[PJM]

The manuals are very vague and I am confused over this situation:

If I set the scan rate to 1000, and the buffer is set to 1000, do i get 1000
data points??

[ej]

The units of "scan rate" is scans per second. Scan rate determines how
quickly the data is taken. If your scan rate is set to 1000 and the
buffer is set to 1000, it takes 1 second for your buffer to fill with
1000 readings.

There is another important property that needs to be set "number of
scans to aquire". It is this property that you use to set the number
of readings that you want to take. This value may be a fixed , or you
can set it to aquire continuously.

note: If your buffer set to a lower value than the number of readings
that you want to aquire you must collect your first readings before
they are overwritten. Depending on the specifications of your DAC,
there are several diffent acquisition techniques that you can use.

Hope this helps,

ej

In article <3991af00
@newsgroups.ni.com>,
"PJM" wrote:
>
> The manuals are very vague and I am confused over this situation:
>
> If I set the scan rate to 1000, and the buffer is set to 1000, do i
get 1000
> data points??
>


Sent via Deja.com http://www.deja.com/
Before you buy.

[PJM]

So, in this case of 1000 scan rate, and 1000 buffer, "one scan" is considered
1000 data points?? Or is a "scan" one data point??

[CraigGraham]

PJM wrote in message
news:3992956d@newsgroups.ni.com...
>
> So, in this case of 1000 scan rate, and 1000 buffer, "one scan" is
considered
> 1000 data points?? Or is a "scan" one data point??

Been a while since I last played with this, so I may be fuzzy.

One scan is a single datapoint from each channel; so if you have one
channel, a scan is one datapoint, if you have 64 channels it's 64
datapoints. The scan rate is how many complete scans are recorded per
second. The buffer is where the data is recorded to. The buffer size is
independent of the scan rate or number of channels.

If you're operating in a mode where you set up the DAQ and perform one
measurement every so often, then the buffer size will probably be 1. As I
reme
mber, the buffer size works in terms of scans, not data points- I may be
wrong on this.

If you're operating in a continuous measurement scenario, then the DAQ
hardware is continuously taking measurements at the rate you specify and
dumping those measurements in the buffer, moving along the buffer as it does
so. When it gets to the end of the buffer, it wraps round to the start and
continues writing data. Your program has to read the new data added to the
buffer before it's over-written; this means the buffer size has to be large
enough to hold all the data that will come in between two iterations of the
program you write to read it. Otherwise the DAQ subvis will generate a
fairly obvious error message so if you're handling your error clusters
(which you are, of course 😉 then you'll see if any timing problems arise.

Be aware that if you're doing many repeated measurements then you shouldn't
be using the "build array" function- this function takes longer and longer
as the array size i
ncreases, and eventually your program is no longer
reading the data quickly enough and you lose measurements.