Campbell Scientific PC400 Datalogger Support Software - Manual (Page 230)

Appendix B. Table-Based Dataloggers
B.3 Table Data Overview
In the datalogger all data is organized into tables with fixed data records. Each
of these tables has a definite number of records that is either fixed by the
datalogger program or allocated when the program is compiled by the
datalogger. Once the maximum number of records for a table has been stored,
the next record stored will overwrite the oldest record in the table. The record
number will continue to increment, and the oldest record will “drop off” the
top.
Tables that are automatically allocated in the datalogger program are allocated
a number of records based on the time interval for the records. The datalogger
attempts to allocate these tables so that all of the automatically allocated tables
fill up at the same time. For example two tables with records stored every 30
minutes and 60 minutes would have twice as many records allocated for the
30-minute table.
Event driven tables should have a fixed size rather than allowing
them to be allocated automatically. If automatically allocated,
event driven tables in the later versions of the operating systems
of CR10X-TD type dataloggers are assumed to have one record
stored per execution interval in calculating the length. NOTE
In CR800, CR1000, CR5000, and CR9000 dataloggers event
tables are assumed to have one record stored per execution
interval.
Since the datalogger normally tries to allocate the table sizes so
they fill up at the same time, if you let the datalogger
automatically allocate table sizes these event driven tables may
take up most of the memory leaving very little for the other,
longer interval, automatically allocated data tables.
Within a data table, data is organized in records and fields. Each row in a table
represents a record and each column represents a field. To understand the
concept of records it may be helpful to consider an example.
Example:
A CR10X-TD is to be used to monitor three thermocouples. Each hour a
temperature for each of the three thermocouples is to be stored. The table has
five fields: DATE_TIME, RECORD #, TEMP1, TEMP2, TEMP3.
The program is written so that each hour an Instruction 84, Table Data,
generates a new “record” in the data table. This hourly table would then be
organized as follows:
DATE_TIME RECORD # TEMP1 TEMP2 TEMP3
2002-01-27 10:00:00 14 23.5 24.6 28.2
2002-01-27 11:00:00 15 24.2 22.4 23.4
Only the hourly data triggered by the Instruction 84 above would be written to
this table. If other table data instructions existed, the output for these tables
would be written to their own tables.
B-6