Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 129)

Section 7. Installation
129
• Size -Table size is the number of records to store in a table before new data
begins overwriting old data. If "10" is entered, 10 records are stored in the
table -- the eleventh record will overwrite the first record. If "-1" is entered,
memory for the table is automatically allocated at the time the program
compiles. Auto allocation is preferred in most applications since the CR800
sizes all tables such that they fill (and begin overwriting the oldest data) at
about the same time. Approximately 2 kB of extra data-table space are
allocated to minimize the possibility of new data overwriting the oldest data
in ring memory when support software collects the oldest data at the same
time new data are written. These extra records are not reported in the Status
table and are not reported to the support software and so are not collected.
CRBasic example Definition and Use of a Data Table
(p. 127)
creates a data table
named OneMin, stores data once a minute as defined by DataInterval(), and
retains the most recent records in SRAM, up to the automatically allocated
memory limit. DataRecordSize entries in the Status table report allocated
memory in terms of number of records the tables hold.
DataInterval() Instruction
DataInterval() instructs the CR800 to both write data records at the specified
interval and to recognize when a record has been skipped. The interval is
independent of the Scan() / NextScan interval; however, it must be a multiple of
the Scan() / NextScan interval.
Sometimes, usually because of a timing issue, program logic prevents a record
from being written. If a record is not written, the CR800 recognizes the omission
as a "lapse" and increments the SkippedRecord counter in the Status table.
Lapses waste significant memory in the data table and may cause the data table to
fill sooner than expected. DataInterval() instruction parameter
Lapses controls
the CR800 response to a lapse. The table DataInterval () Lapse Parameter
Options
(p. 130)
lists Lapses parameter options and associated functions.
Note Program logic that results in lapses includes scan intervals inadequate to the
length of the program (skipped scans), the use of DataInterval() in event-driven
data tables, logic that directs program execution around the CallTable()
instruction.
A data table consists of successive 1-kB data frames. Each data frame contains a
time stamp, frame number, and one or more records. By default, a time stamp and
record number are not stored with each record. Rather, the data extraction
software uses the frame time stamp and frame number to time stamp and number
each record when it is stored to computer memory. This technique saves
telecommunications bandwidth and 16 bytes of CR800 memory per record.
However, when a record is skipped, or several records are skipped contiguously, a
lapse occurs, the SkippedRecords status entry is incremented, and a 16-byte sub-
header with time stamp and record number is inserted into the data frame before
the next record is written. Consequently, programs that lapse frequently waste
significant memory.
If Lapses is set to an argument of 20 , the memory allocated for the data table is
increased by enough memory to accommodate 20 sub-headers (320 bytes). If
more than 20 lapses occur, the actual number of records that are written to the
data table before the oldest is overwritten (ring memory) may be less than what
was specified in the DataTable().