U3 : on the fly streaming / polling / streaming / polling

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For the sake of clarity, I am re-posting here an important U3 development : streaming.

Many thanks to DAQFactory support.

Many thanks to LABJack support.

It is now clear how U3 can, using streaming under DAQFactory, acquire data with a 4,000 Hz sampling frequency per channel. Meaning a 40,000 Hz banwidth with 10 such channels. Amazing !

And since the beginning, it is also clear how U3 can, using polling, acquire data with a 100 Hz sampling frequency per channel.

There has been one post asking how to setup a system working with fast signals (hence needing the streaming mode) and working also with slow signals (hence needing the polling scheme). Both together.

Three answers are provided :

1 - Use streaming for all signals including the slow ones, then decimate or average the output for the slow ones. This may be unconvenient because you waste bandwith and you cannot achieve the fastest sampling frequency on the two or three fast signals you may have. You are going to divide the max available bandwith by too much.

2 - Use two U3 modules. First one in streaming for fast signals, second one in polling for slow signals. This may be unconvenient because you may need to acquire two fast signals plus eight slow signals, then you realize that you have many unused inputs with the two U3 units. Qualitatively, this is the best method as you can achieve a continuous sampling without gaps. For all inputs. This is of importance compared with method 3.

3 - dynamically configure the U3 mode. Only one U3. Dealing with the fast signals, in streaming during 0.2 second. Hence aquiring something like 500 points (fast channels). Then just after, reverting to the polling mode, and aquiring the other channels, much slower, at a 20 Hz sampling frequency per channel. Doing this during 0.5 sec (hence aquiring 10 points per channel) . Then reverting to the fast mode (streaming on fast channels) ... then reverting to the slow mode (polling on the slow channels), and so on ... The advantage of this method is that you can manage the U3 bandwidth at your will.


For pure efficiency, I have a preference for method 3.

Is it possible to get a documented .CTL file putting this scheme at work ?

I think many U3 users will be interested in applying the U3 streaming + polling alternating sequence.

How to generate the configuration parameters like requesting the streaming channels (what changes if one wants 2, 3, 4, 5, 6, 7 ... streaming channels) ? Is there any documentation available (sending configuration words) ?


from Brussels

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