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Section 2. Data Storage and Retrieval

The CR800 can store individual measurements or it may use its extensive processing

capabilities to calculate averages, maxima, minima, histograms, FFTs, etc., on periodic or

conditional intervals. Data are stored in tables. The number of tables and the values to

output in each table are selected when running Short Cut (Overview) or when writing a

datalogger program directly (Sections 4 – 9).

2.1 Data Storage in CR800

2.1.1 Internal SRAM

Internal SRAM is used as the storage area for a data table. The maximum

number of data tables that can be created is 30. Internal SRAM is battery

backed. Data remain in memory when the CR800 is powered down. Data in

SRAM are erased when a different program is loaded and run.

There are 4 Mbytes of SRAM. Some of this is used by the operating system

and for program storage. The rest is available for data storage. When a new

program is compiled, the CR800 checks that there is adequate space in SRAM

for the data tables; a program that requests more space than is available will

not run.

In September 2007, Campbell Scientific began increasing the

SRAM size from 2 MB to 4 MB. Dataloggers with a serial

number greater than or equal to 3605 will have a 4 MB SRAM.

The 4 MB dataloggers will also have a sticker stating “4M

Memory”.

NOTE

2.2 Internal Data Format

TABLE 2.2-1 CR800 DATA TYPES

Data Type Size Range Resolution

LONG 4 bytes -2,147,483,648 to +2,147,483,647 1 bit (1)

IEEE4 4 bytes 1.8 E -38 to 1.7 E 38 24 bits (about 7 digits)

FP2 2 bytes -7999 to +7999 13 bits (about 4 digits)

Boolean 4 bytes True/False —

String 1 byte per character

+ 1 byte, 16 byte minimum

— —

Data are stored internally in a binary format. Variables and calculations are

performed internally in IEEE 4 byte floating point with some operations

calculated in double precision. There are six data types used to store data. The

data format is selected in the instruction that outputs the data. Within the

CR800, time is stored as integer seconds and nanoseconds into the second

since midnight, the start of 1990. While IEEE 4 byte floating point is used for

variables and internal calculations, FP2 is adequate for most stored data.

Campbell Scientific 2 byte floating point provides 3 or 4 significant digits of

2-1

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Campbell CR800 Spezifikationen Seite 69