Campbell Scientific NL115 Ethernet Interface and CompactFlash Module - Manual (Page 29)

NL115 Ethernet and CompactFlash ® Module
7.4 CF Card Data-Retrieval Details
Data stored on CF cards can be retrieved through a communication link to the
datalogger or by removing the card and carrying it to a computer.
7.4.1 Via a Communication Link
Data can be transferred to a computer via a communication link using one of
Campbell Scientific’s datalogger support software packages (for example,
PC200W, PC400, LoggerNet). There is no need to distinguish whether the
data is to be collected from the CPU memory or a CF card. The software
package will look for data in both the CPU memory and the CF card.
The datalogger manages data on a CF card as final-storage table data, accessing
the card as needed to fill data-collection requests initiated with the Collect
button in datalogger support software. If desired, binary data can be collected
using the File Control utility in datalogger support software. Before collecting
data this way, stop the datalogger program to ensure data are not written to the
CF card while data are retrieved. Otherwise, data corruption and confusion
will result.
7.4.1.1 Fast Storage/Data-Collection Constraints
When LoggerNet collects data from ring tables that have filled, there is the
possibility of missing records due to the collection process. LoggerNet uses a
“round-robin” collection algorithm that collects data from multiple tables in
small blocks as it sequences around to all the tables. Collection starts at the
oldest data for each table. When a ring table has filled, the oldest data is
overwritten by current data.
With filled ring tables, as collection begins LoggerNet queries the datalogger
for the oldest data starting with the first table. When this data block is
returned, LoggerNet goes to the next table and so on until all of the tables are
initially collected. By the time LoggerNet makes the second pass requesting
more data from the tables, the possibility exists that some of that data may have
been overwritten, depending on how fast the datalogger is storing data (that is,
data storage rate, number of table values, and number of tables).
Normally, LoggerNet gets ahead of the storing datalogger and the remaining
data is collected without gaps; however, if the datalogger is storing data fast
enough, it is possible to get into an always-behind scenario where LoggerNet
never catches up and the datalogger repeatedly overwrites uncollected data.
The possibility of missing records is greater when collecting data via IP. This
is due to the high demand of IP on processor time. The risk is greatest with a
CR1000 datalogger using IP, because of its slower processor speed relative to
the CR3000.
7.4.2 Transporting CF Card to Computer
With large files, transferring the CF card to a computer may be faster than
collecting the data over a link.
Removing a card while it is active can cause garbled data
and can actually damage the card. Do not switch off the
datalogger power if a card is present and active.
CAUTION
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