csmstr - Omega Engineering

CONFIGURING DATA LOGGING
THE LOGGING PROCESS
most likely be different, however, each file will represent a fix length of time regardless of the
number of samples.
THE LOGGING PROCESS
Crimson’s data logger operates using two separate processes. The first samples each data
point at the rate specified in its properties, and places the logged data into a buffer within the
RAM of the Master module. The second process executes every two minutes, and writes the
data from RAM to the CompactFlash card. This structure has several advantages…
• Writes to the CompactFlash card are guaranteed to begin only on a two-minute
boundary—that is, at exactly 2, 4 or 6 minutes past the hour, and so on. This
means that if your Master module supports hot-swapping of CF cards, you can
wait for the next burst of writes to start, and, when the CompactFlash activity
LED on the front of the Master ceases to flicker, you are guaranteed to have until
the start of the next two-minute interval before further writes will be attempted.
This means that you can remove the card without fear of data corruptions. As long
as you insert a new card before four minutes have elapsed, no data will be lost.
• Writes to the CompactFlash achieve a much higher level of performance, by
avoiding the need to continually update the card’s file system data structures for
every single sample. For logs configured to sample at very high data rates, the
bandwidth of a typical CompactFlash card would not allow data to be written
reliably in the absence of such a buffering process.
Note that because data is not committed to CompactFlash for up to two minutes, up to this
amount of log data may be lost when the Master is powered-down. Further, if the Master is
powered-down while a write is in progress, the CompactFlash card may be corrupted. To
ensure that such corruption is not permanent, the Master module uses a journaling system that
caches writes to additional non-volatile memory within the Master. If the Master detects that
a write was interrupted during power-down, the write will be repeated when power is
reapplied, thereby reversing any corruption, and repairing the CompactFlash card.
This means that if you want to remove a CompactFlash card from a Master performing data
logging, you must observe the procedure described above with respect to the activity LED,
and only remove power when the activity has ceased. If you are not sure if the Master was
powered-down correctly, reapply power, allow a CompactFlash write sequence to complete,
and power down according to the correct procedure. The card can then be removed safely.
Since the gyrations required to remove a CompactFlash card are somewhat complex, Crimson
provides two other mechanisms for accessing log files, thereby eliminating the need for such
removals. These methods are described below.
ACCESSING LOG FILES
There are three additional methods of accessing log files…
• The less preferable method is to mount the card as a drive on a PC via the
process described at the start of this manual, so that the logs can be copied using
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