Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 129

Section 7. Installation • 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 subheader 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(). 129