Campbell Scientific LoggerNet Datalogger Support Software - Manual (Page 530)

Appendix D. Software Organization
the last poll. The tables to be collected are specified by the user in the Setup
Screen.
As each record is written to the data cache, the server adds a filemark number
to the record as it is stored. This filemark number is used to identify
discontinuities in the data. The filemark number starts out as zero when the
table for the data cache is created or re-initialized. This number is incremented
each time a discontinuity is seen in the data records. Such a discontinuity can
occur when there is a gap in the record numbers because the data table filled
and overwrote the requested data. This also can occur if the record number
rolls over from the maximum to start back at zero or an identical program is
loaded into the datalogger without going through the server.
Data can also be collected from the datalogger using a manual poll operation.
This is achieved by selecting Collect Now from the Connect Screen. When a
manual poll is done the data from the datalogger is saved in the output data file
and is also put into the data cache.
D.2.3 Retrieving Data from the Cache
Once the data has been stored in the data cache it is retrieved by the
applications such as the graphical and numerical displays that request the data
by datalogger, table or array, and data field. The data can be requested by a
query where the request specifies the starting and ending timestamp or record
number along with the data to retrieve.
D.2.4 Updating Table Definitions
When the table definitions are obtained from the datalogger they are kept in the
server and used to identify the data available in the data cache. Every time new
data is collected from a datalogger, a table definition signature is sent that
should match the signature stored in the server. If this table definition
signature doesn’t match it indicates that the table definitions in the datalogger
have changed.
There are a number of things that could cause datalogger table definitions to
change. A new program may have been downloaded to the datalogger, or the
keyboard display may have been used to manually make changes to the
datalogger program.
If the datalogger program is re-compiled without changing table
definitions, the record numbers will reset to zero causing the
server to assume the datalogger record numbers have wrapped
around. This will result in the re-collection of all of the data in the
datalogger.
When a change in table definitions is detected, the server stops data collection
and indicates in the Collection State of Status Monitor that the table definitions
have been changed. Data collection cannot be restarted until either a new
datalogger program is loaded into the datalogger by the server, or updated table
definitions are received from the datalogger. Either of these actions causes the
data in the data cache for that datalogger to be removed and new data cache
tables set up based on the new table definitions for that datalogger. LoggerNet
will save the existing output data file with a modified name and create a new
output data file.
NOTE
D-2