Campbell Hausfeld SM4M - Manual (Page 21)

4-1
Section 4. Storage Module Memory:
Details of Data Storage
Detailed knowledge of the way data is stored in the storage module is not normally necessary
for day-to-day use. However, if you ever need to calculate and utilise the exact memory
available, or try to extract corrupted data, it may be helpful to know these details.
4.1 File Marks
File Marks are used to separate data in the Storage Module. A File Mark is
automatically placed in the SM’s memory when the SM is connected to a power
source (datalogger or retrieval interface) or when the datalogger compiles a
program containing Instruction 96. For example, if you retrieve data from one
datalogger, disconnect the SM and then connect it to a second datalogger, a File
Mark is placed in the data when the second datalogger is connected. This mark
follows the data from the first datalogger and precedes the data from the second.
A File Mark can also be inserted under program control.
The SM must be disconnected from power for a minimum of
10 seconds to be certain it places a File Mark in the SM when
reconnected.
A File Mark can be placed in the data by using SMS (part of PC208W), the *9
Mode command 3 (not in 21X or CR7) or from a special P96 command in the
datalogger program.
The Storage Module will not store back-to-back file marks. If a File Mark is the
last thing stored in memory, a second File Mark will not be stored.
4.2 Pointers
The SM has several internal pointers which are used to keep track of data for both
storage and retrieval operations.
The Display Location Pointer (DLP) is used to display Storage Module data.
Ordinarily, when the SM is connected to a datalogger, the DLP will be at the
beginning of the current file. The DLP can be positioned anywhere within the
data, using the nnnnnnG command (see Appendix B). Data can then be ‘dumped’
to the user device using one of the appropriate commands ( nnnnC or nnnnF). The
dump will begin at the DLP and continue until it reaches the Storage Reference
Pointer (SRP) or until the next File Mark.
The Storage Reference Pointer (SRP) indicates the next location to be written to
in the Storage Module. It will always be at the end of data in the Storage Module.
The Dump Pointer is an internal Storage Module pointer used for keeping track
of the current start-of-dump for Storage Module to Storage Module data dumps
and for the data retrieval options of the SMS program. The user can move the
Dump Pointer with the 4H command (see Appendix B). The Dump Pointer marks
the end point of the previous data dump and thus the start-of-dump location for
the next dump. Moving this pointer ‘loses’ this reference point, which can cause
problems with subsequent dumps.
CAUTION