Analog Outputs stops (without reason) ?!

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Tobias on 14 Dec 2011
Hey everybody!
I hope one of you can help me!
I am working on a Programm using DAQ-Toolbox and a National Instruments I/O USB Card. I am trying to output data continuously by using an analog outputobject. It got a Samplerate of 100 and 100 Values get enqueued before the object is started. After start a timerfunction enqueues new Data by using "putdata". The first 100 Samples are output fine but than the object stops though there are still 400 Values in the queue. It stops without any error message.
Why does the object stop?
In my opinion the object should run until no more data is left in the queue... .
If you need further information for an answer just let me know!

Answers (3)

Walter Roberson
Walter Roberson on 14 Dec 2011
Usually USB buffers data until the USB packet size (close to 1000 bytes) is met. If your data is 2 bytes per sample, your buffer is getting drained until less than ~1000 bytes is left.
USB is not suitable for continual output of short packets -- at least not without getting in to details of USB that Mathworks has never exposed the user to.
  1 Comment
Tobias on 14 Dec 2011
Hey Walter!
Thanks for caring about my problem!
I dont think that it is a USB-problem because I use the following line to show in the command window how much Data is already IN the Hardware ready to be send.
And this command shows before the analog object stops that 400 Values are queued and waiting for being output. After the object stopped this property is set to zero.
So, what do you think?

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Chirag Gupta
Chirag Gupta on 14 Dec 2011
Can you post bit of the code? Also are you using the new Session based interface (R2011b) or the legacy interface ?
If you are using the legacy interface (analogoutput object) then you might need to set this property:
  1 Comment
Tobias on 14 Dec 2011
Hey Chirag!
Also thanks for caring about my problem!
I am using Matlab 2011. I can`t specify which version in detail at the moment as I am at home and not at college.
The problem occours in the part of the programm where I use the legacy-inferface. I am not using the session-based-interface in this part of the programm as I need continuous data to create a "Liveplot" of a stepresponse.
I think the session-based-aquisition isn`t made for getting continuously data for a "liveplot" or am I wrong?
I ve read already the documentation about "repeatoutput" and understood it that way that this function takes the already enqueued data an outputs it in a loop so many times as you enter. That makes this function worthless to me because I musst be able to enqueue NEW data due to the timer function.
So here is my code:
%% Declare continuous analog Output
% Outputobjekt erzeugen
%Outputkanäle an Objekt anfügen
addchannel(gDyn_ao,1); % Heizer
%Sprung von
%Initialen Datensatz erzeugen
for h=2:gDyn_sampleRate
firstData(h)=gDyn_bis; %Heizer
%TimerfunctionCallback deklarieren und ersten Datensatz übergeben
%Set Time to trigger TimerfunctionCallback
%Schreibrate setzen
% Ersten Daten an analoges Device übergeben
%% Starte kontinuierliche, analoge Objekte
%Globale Variablen reseten
%Input- und Outputobjekt zusammenfassen
gDyn_analogObjects=[gDyn_ai gDyn_ao];
%I/O aktivieren
And the timerfunction:
% AnalogOUT-TimerCallback: getriggert neue Daten in den OutputPuffer schieben
function ao_enqueueMoreData(hObj,event,handles)
global gDyn_sampleRate;
global gDyn_von;
global gDyn_bis;
global zaehler;
%if (hObj.SamplesAvailable+lenth(nextData))>=hObj.MaxSamplesQueued
%if (hObj.SamplesAvailable+gDyn_sampleRate)>=hObj.MaxSamplesQueued
if hObj.SamplesAvailable>=gDyn_sampleRate*4
%generiere neue Outputdaten
for k=1:gDyn_sampleRate
nextData(k)=gDyn_bis; %Heizer
%Schiebe neue Daten in Puffer
Can you find any mistakes ?

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Chirag Gupta
Chirag Gupta on 14 Dec 2011
Looking at the documentation, it states that all the data must be queued using putdata before starting the analogoutput object.
So the TimerFcn will not work, unless you do the following: a) Change the TriggerType to Manual, b) and retrigger after putting more data.
A more convenient solution would be offered by Session based architecture which is perfect for this:
I am assuming you are doing both input and output simultaneoulsy! You can use Background operations to continuously queue data to to be output (using a DataRequired callback), example:
You can also do both Input and Output simultaneously and do a live plot using 'DataAvailable' for inputs! (You might want to use startBackground(), if you want the output data to happen in the background)

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