Black Box IEEE 1284 Podręcznik Użytkownika Pobierz pdf (Strona 23)

The Black Box Toolkit Page 23 of 98

7. CALIBRATION

7.1. Assessing the suitability of a host PC for realtime data collection

Before you use the toolkit with a new host PC you should run the calibration utility. You are

also advised to run it from time to time to check that the host is still capable of a high

enough sampling rate to support sub-millisecond accuracy.

The calibration utility takes 500,000 samples and reports on various aspects related to

sampling performance and consistency. You are free to alter the number of samples and

the priority with which the calibration utility runs. A typical PC will take around 10 seconds

to complete 500,000 samples with realtime thread priority. You should be careful not to

enter a too higher number of samples as you will not regain control of your PC until

sampling has finished. Remember as the BBTK runs with realtime thread scheduling the

mouse, keyboard and other peripherals may not function during calibration. You should

also note that approximately 1-2Mb of hard drive space will be used for each second of run

time.

To start calibrating your host PC click on the “Calibrate” button. Once calibration is

underway an hourglass icon will be displayed. When all 500,000 samples have been taken

a status bar will indicate the progress of the data analysis. Once all data has been

processed the upper spreadsheet will be filled-in and a plot showing tick variance per

sample will be plotted. The higher the number of ticks per sample and the larger the

variation the poorer the performance of the host PC for realtime work. Should the PC tick

variance be too high or the sampling rate too low you are advised not to use the toolkit

with the PC in question. The screen shot below shows a typical PC’s performance when

running with realtime thread scheduling. The PC in question is a 1.4Ghz AMD Duron with

1Gb of RAM. It is heavily loaded with software and is running many background

processes. Note the scale on the Y axis and the regular interruptions. These are typically

caused by low level operating system tasks or unruly software. Remember lower and more

consistent peaks are better.

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Black Box IEEE 1284 Podręcznik Użytkownika Strona 23