Campbell CR6 Series, Operator'S Manual

Page: 205 / 690

Section 7. Installation
205
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 datalogger
support software
(p. 97)
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.
Rules on table size change if a CardOut() instruction or TableFile()
instruction with Option 64 are included in the table declaration. These
instructions support writing of data to a memory card. Writing data to a card
requires additional memory be allocated as a data copy buffer. The CR6
automatically determines the size the buffer needs to be. See Memory Cards
and Record Numbers
(p. 500).
CRBasic example Declaration and Use of a Data Table
(p. 203)
creates a data table
named OneMin , stores data once a minute as defined by DataInterval() , and
retains the most recent records in SRAM. DataRecordSize entries in the
DataTableInformation table report allocated memory in terms of number of
records the tables hold.
DataInterval() Instruction
DataInterval() instructs the CR6 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 CR6 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
CR6 response to a lapse. See table DataInterval () Lapse Parameter Options
(p.
206)
for more information.
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, and 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 datalogger support
software data extraction extraction routine uses the frame time stamp and frame
number to time stamp and number each record as it is stored to computer memory.
This technique saves comms bandwidth and 16 bytes of CR6 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.