Campbell Scientific CR7 Measurement and Control System - Manual (Page 40)

SECTION 2. INTERNAL DATA STORAGE
2-2
The Data Storage Pointer (DSP) is used to
determine where to store each new data point in
the Final Storage area. The DSP advances to
the next available memory location after each
new data point is stored.
The DPTR is used to recall data to the LCD
display. The positioning of this pointer and data
recall are controlled from the keyboard (*7
Mode).
The PPTR is used to control data transmission
to a printer, Storage Module, or other serial
device. Whenever on-line printer transfer is
activated (*4 Mode or Instruction 96), data
between the PPTR and DSP are transmitted.
When on-line transfer to a SM192/716 Storage
Module is activated by Instruction 96 with output
code 30, data is transmitted each time an
output array is stored in Final Storage IF THE
STORAGE MODULE IS CONNECTED TO THE
CR7. If the Storage Module is not connected,
the CR7 does not transmit the data nor does it
advance the PPTR to the new DSP location. It
saves the data until the Storage Module is
connected. Then, during the next execution of
Instruction 96, the CR7 outputs all of the data
between the PPTR and the DSP and updates
the PPTR to the DSP location (Section 4.1)
The MPTR is used in transmitting data over a
telecommunications interface. When
Telecommunications is first entered, the MPTR
is set to the same location as the DSP.
Positioning of the MPTR is then controlled by
commands from the external calling device
(Section 5.1).
NOTE: All memory pointers are set to the
DSP location when the datalogger compiles
a program. For this reason, ALWAYS
RETRIEVE UNCOLLECTED DATA
BEFORE MAKING PROGRAM CHANGES.
2.2 DATA OUTPUT FORMAT AND
RANGE LIMITS
Data are stored internally in Campbell
Scientific's Final Storage Format (Appendix
C.2). Data may be sent to Final Storage in
either LOW RESOLUTION or HIGH
RESOLUTION format. Low resolution is the
default. To change the resolution, Instruction
78 (Section 11) must precede the Output
Instructions in the program table.
2.2.1 RESOLUTION AND RANGE LIMITS
Low resolution data is a 2 byte format with 3 or
4 significant digits and a maximum magnitude
of ±6999. High resolution is a 4 byte format
with 5 significant digits and a maximum possible
output value of ±99999 (see Table 2.2-1 below).
TABLE 2.2-1. Resolution Range Limits of
CR7 Data
Minimum Maximum
Resolution Zero Magnitude Magnitude
Low 0.000 ±0.001 ±6999.
High 0.0000 ±.00001 ±99999.
The resolution of the low resolution format is
reduced to 3 significant digits when the first (left
most) digit is 7 or greater (Table 2.2-2). Thus, it
may be necessary to use high resolution output
or an offset to maintain the desired resolution of
a measurement. For example, if water level is
to be measured and output to the nearest 0.01
foot, the level must be less than 70 feet for low
resolution output to display the 0.01 foot
increment. If the water level is expected to
range from 50 to 80 feet the data could either
be output in high resolution or could be offset by
20 feet (transforming the range to 30 to 60 feet).