How to achieve individual analog input and analog output rates with DASYLab.

DASYLab has several Measurement driver options for timing a measurement. Each enabled driver has one or more time bases associated with installed devices. Depending on the device, it may have both hardware(device timed) and software (PC timed) time bases. These are clocks that are used to timestamp the data that is being acquired. You should usually use the hardware time base for analog inputs, as it is very accurate and fast.However, you need to understand how DASYLab and your devices work when you need to also do analog outputs.

Some devices have multiple subsystems. For example, the Measurement Computing USB-234 has two input subsystems, one for Analog Inputs and one for Digital (discrete) inputs. It has two output subsystems, one for Analog Outputs and one for Digital (discrete) outputs.

If you have both analog inputs and outputs, and the analog output data is paced by the analog input, the DASYLab worksheet setup is limited by the slowest device subsystem. In most cases, analog outputs are not able to update as fast as the analog inputs. In order for DASYLab to output to an Analog Output, the data arriving at the Analog Output module has to have timing characteristics that are compatible with the Analog Output subsystem of the device.

The reason for this is that the DASYLab Analog outputs use the scan rate and block size from the data source that is connected to the module inputs. Consequently, if the source is set to a rate that is faster than the supported rate of the analog output,, DASYLab will display an error message similar to the one shown below.  

For example, the Measurement Computing USB-234 has both analog inputs and outputs. The analog inputs have a maximum scan rate of100kS/sec but the analog outputs only have a maximum update rate of 5kS/sec. If you select the hardware paced time base of the USB-234 for both the AI and AO operations you are limited to 5Ks/sec for both inputs and outputs. Hence, the error message that is displayed above.

To avoid this error while scanning the analog inputs at the maximum hardware paced time base, and outputting data to the analog outputs at the full hardware paced output speed, you will need to use different time bases for the inputs and outputs.

For this case you would use the USB-234’s hardware paced time base for the Analog Input modules and use the “Driver” time base or the “DASYLab” time base for the Analog Output modules. This will allow you to run both at the irrespective rates.

Other techniques include

-       Down sample the input data to a supported rate for the AO by using the Average or Separate modules

-       Generate data for the AO using a software generator module, such as the Generator,Switch, Slider or Read Global variable. Configure the module so that the Options/Synchronization uses an appropriately configured time base (DASYLab,Driver).