continuous data acquisition with external trigger NI-USB 6218

Hi,

I am trying to acquire continuous analog input data from a NI-USB 6218. I want to start and stop acquisition when I receive an external trigger on another channel (either analog or digital).

So far I've tried with the digital trigger using the following code:

set(ai,'TriggerType','HwDigital')
set(ai,'HwDigitalTriggerSource', 'PFI0')
set(ai,'TriggerCondition','PositiveEdge')
set(ai,'TriggerConditionValue',4.5)

set(ai,'SampleRate',RATE)
set(ai,'SamplesPerTrigger',Inf)
set(ai,'InputType','SingleEnded');
set(ai,'TimerFcn',@AcquireData);
set(ai,'TimerPeriod',0.5);

function AcquireData(hObject, event)
persistent totalData;
data = getdata(hObject);
if isempty(totalData)
    totalData = data;
else
    totalData = [totalData; data];
end

The problem is that as I run the command "start(ai)" I get an error:

Error using daq.analoginput_nidaqmx_1_87/start
NIDAQmx error : Requested value is not a supported value for this property. The
property value may be invalid because it conflicts with another property.
Property: DAQmx_SampQuant_SampPerChan Requested Value: 0 Valid Values Begin
with: 2 Valid Values End with: 8589934590  Task Name: MWDAT0183  Status Code:
-200077

It looks like I cannot set the samplePerTrigger property to Inf...

Can anyone help? Any suggestion on how to start/stop ai continuous acquisition with an external trigger? I need a hardware trigger because I have to start acquisition at the very same time of trigger turning on.

Thanks, Marianna