Data synchronisation point

Many systans are backed up intermittently to high capacity removable drives dedicated to this purpose. The drive may then be physically transported to an archive which is geographically distant from the primary data centre. This process of backup and transport takes time and effort and the fact that it is undertaken intermittently means that the snapshot of data will be out of date the moment it is created. Should restoration of data be required it is only ever possible to reinstate the system to a point in the past. Transactions which have taken place since the backup was created run the risk of being irretrievable. The term data synchronisation point represents the absolute point in time when the data snapshot was taken. After restoration of the system the state of affairs would resemble that at the synchronisation point The further in the past the synchronisation point, the more data is permanently lost... 

In fast photochemical reactions the measured data will have some time lag which is determined by the speed of the wavelength drive. Then, since synchronisation has to relay to just a few supporting points, interpolation becomes erroneous. Nevertheless this program must be chosen when dealing with thermal reactions or with the examination of photoreactions with superimposed thermal reactions. 

Fig. 4.6. Schematic representation of absorbance/transmittance versus time curves. The symbols give the data points for three wavelengths, measured r tively one after another. Synchronisation is obtained by taking int polated values at equal times (perpendicular lines).

Both the monochromators are adjusted to the same wavelength, the reference is taken, and the absorbance of the sample is measured. Afterwards the next wavelength is selected. In this type of measurement the irradiation times differ for the various wavelengths. Therefore the data points have to be synchronised as explained in Fig. 4.6. 

In both cases the data acquisition in the TCSPC channels is synchronised with the scanning by clock pulses from the scan controller. It must, however, be taken into account that the length of the lines of the scan varies since the return points of the scan are controlled by the detector overload signals. Therefore, the scan software must store the positions of the return points and the number of pixels between. These positions are used later to adjust the lines horizontally. 

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