daq code storing data problem

i wrote this code to acquire data from usb6008 and store it when i used just two channels or three and set s.duration =20 it enters a loop of reading and without end however i disconnect the daq

if true
  % % Initialization

global Temperature global Irradiance global current global Vref global data

%%get connected devices d = daq.getDevices %create session s = daq.createSession('ni') s1= daq.createSession('ni') %add analog channel s.addAnalogInputChannel('ID',channel num, 'measurement type') s.addAnalogInputChannel('Dev1','ai0', 'Voltage')%irradiance % s.addAnalogInputChannel('Dev1','ai3', 'Voltage')%Temperature s.addAnalogInputChannel('Dev1','ai6', 'Voltage')%current s1.addAnalogOutputChannel('Dev1', 'ao0', 'Voltage') % set rate of scan 1000 scans/second , run for 30 seconds

 s.Rate=100;
s.DurationInSeconds =20 
%v.TerminalConfig = ' Differential';
v.TerminalConfig = ' SingleEnded';
%_____________________________
%aquire data and process
lh = s.addlistener('DataAvailable', @plotData);
%store data
h = s.addlistener('DataAvailable', @storedata);
% Analog Output 
ao_value = 2.74;
s1.outputSingleScan(ao_value)

s.startBackground() s.wait()

 delete (lh)
 delete (h)

 storing code 
    % 
function storedata(src,event)
    persistent tempData;
    global data 
    if(isempty(tempData))
         tempData = [];
     end
     plot(event.TimeStamps, event.Data)
     tempData = [tempData;event.Data];
     data = tempData
     xlswrite('collect.xls',  data);