can't set sample rate via data aquition tool box

I fails to set sample rate for my data acquision hardware.

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current matlab version
2012a 32bit
OS version
Windows 7, 64 bit
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However I could set the samplerate successfully in old computer

matlab version
2007a, 32 bit
windows xp, 32 bit
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Here is my code.

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the hardware infomation is listed below.
***************************************

out=daqhwinfo(ao)

out = 

                  AdaptorName: 'dtol'
                         Bits: 16
                   ChannelIDs: [0 1]
                     Coupling: {'DC Coupled'}
                   DeviceName: 'DT9804(00) : DT9804'
                           ID: '0'
                MaxSampleRate: 500
                MinSampleRate: 2.0000e-04
               NativeDataType: 'uint16'
                 OutputRanges: [-10 10]
                     Polarity: {'Bipolar'}
                   SampleType: 'Scanning'
                SubsystemType: 'AnalogOutput'
                TotalChannels: 2
      VendorDriverDescription: 'Data Translation'
          VendorDriverVersion: 'V6.7.4.8'

************************************************
the samplerate property is listed below
*************************************************

propinfo(ao,'samplerate')

ans = 

                 Type: 'double'
           Constraint: 'bounded'
      ConstraintValue: [2.0000e-04 500]
         DefaultValue: 100
             ReadOnly: 'whileRunning'
       DeviceSpecific: 0

******************************************* when I set sample rate, warning shows up ****************************************

set(ao,'samplerate',128)

Warning: This hardware could not support the requested value of
128.00 for SampleRate.  SampleRate has been set to 100.00.

**************************************************************

any idea?

Thanks

Tao

[Walter Roberson]

What are the units for that samplerate ?