List of the hardware supported is the same as for the internal pulse generators
All necessary parameters for the acquisition boards are already specified in the Lap_sys.ini file during the installation process.
- calibration and configuration of the National Instruments NI6251 AD/DA card
Starting from the software version 3.3 new drivers for the NI6251 card are provided. They allow to calibrate and configure the card specifically for a given computer on which the measurements are performed. Each acquisition card is characterised by the so-called one transient dead time. This is a time which a computer needs to acquire one measured transient, average it with the previous ones, reprogram the card for the next transient and finally to trigger the next measurement. Usually this time is of an order of milliseconds. The new card drivers use the card hardware clocks to perform the measurements with one fixed dead time for all transients. This measurement mode allows eliminating the trigger jitter usually present if the system forces the card to wait with the next trigger until other system activity finishes. If the fixed card dead time is too short and the system activity makes that the transient acquisition is not finished in the moment of the next trigger command then this transient is lost and not considered in the transient averaging procedure. We called this transient the lost loop.
The current driver allows finding via a calibration procedure the shortest dead time for which a number of lost loops is acceptable and is of an order of few percent. This dead time is the best possible choice for a given computer, its processor speed, an activity of other resident programs, etc. At the start-up of the main program during the initialisation procedure the card is calibrated and the best dead time is calculated. The result of the current calibration procedure and the average value of the previous procedures can be viewed on the NI card calibration/configuration form. This form can be invoked from the Hardware configuration window or from the Transient Processor window Configuration menu when the NI card is chosen for the transient acquisition.
In the calibration form the current, average, or manual values of the transient dead time can be chosen to be used for the measurements. The efficiency of the current choice can be inspected by viewing the number of lost loops for the dead time used. The number of measured and lost loops in the last measurement and the sum of the loops in all measurements since last hardware initialisation are shown. Our experience shows that the number of lost loops less than, say, 2% is acceptable. The warning level for the lost loops can be set and if this level is exceeded then a warning sign on the status bar of the Transient Processor window is shown. The best choice for the dead time is as short as possible for which the percentage of lost loops is less than 1-2%. For safety, i.e. to avoid a case of unforeseen processor activity affecting the measurement, the dead time can be made systematically longer than obtained from the calibration by around 10%.
Setting the dead time to 0 manually turns off the hardware measurement loop timing clock. This results in a significant trigger jitter as the acquisition loop runs as fast as possible experiencing different dead time in each pass, depending on processor load and computer other activities.
NOTE: An inexperienced user is advised to use the average value of the dead time at the start-up and to make this time longer by 10% for safety.
Finally, the calibration form allows choosing the moment at which the synchronisation edge (a rising or falling step from 0 to 5V) is applied. This edge initiates the transient sampling and then acquisition. For typical conventional or Laplace measurements it is advised to set this edge to “after all pulses”. Then the filling (1 or 2) or injecting (1 prime) pulses are not seen at the beginning of the transient. For non-standard measurements this edge can be set in the same moment as beginning of the filling pulse (at time”0”) or after the filling pulse but at the beginning of the injection pulse (“after pulse 1”). If the injection pulse is not used the first two choices are equivalent.
The positive (default) or negative convention of the synchronization pulse can be used. Positive means that active level of synchronization pulse is high (+5V) and its rising edge is used to start acquisition. Negative means the opposite: active level is low (0V) and falling edge is used.
NOTE: the last used choices for the synchronization edge are also used during the next program start-up.