FIS HDL1, Руководство по эксплуатации

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PROBLEMS WITH FAST LOGGING
In theory, it is possible to log data as fast as every second, but this is usually not
achievable in practice.
The most important limitation is the time it takes for SDI sensors to return their
readings. Since these are all read sequentially the logging interval should not be
shorter than the worst case time for all sensors to return their readings. This may be
available from the manufacturer s literature.
The data logger takes some time itself to process the DLP and store the data. The
time it takes varies with the complexity of the program and the amount of data to be
stored, but is not normally more than a few seconds.
NOTE: If an attempt is made to log data faster than the data logger can get it from
the sensors, then there will appear to be holes in the data.
As an example, if you try to log every 10 seconds with a sensor that takes 12
seconds to return a reading, you will store readings at 10s, 30s, and so on instead of
0, 10, 20... This may also occur if there is a very slow SDI sensor that is only read
occasionally. If for example, it took 50 seconds to return a reading, then 5 of the 10
second SDI readings will be missing because the SDI bus cannot be used for
anything else while waiting for the slow sensor.
This is a limitation of the SDI 1.1 bus standard.
CALCULATION OF DATA STORAGE FILL TIME
The worst case data storage requirement for any sensor is 3 bytes, with 2 bytes
being about average.
For a simple example, assume that 5 readings are logged every 15 minutes as a
single LOG() command.
5 readings needs 15 bytes of storage (5 x 3 = 15)
Add 5 bytes for time tag and set number (15 + 5 = 20 bytes)
Divide this into 507904 (the total amount of storage available) to
get 25,395 sets of readings.
25,395 sets 15 minutes apart is 380,925 minutes or 264 days.
This means that you must recover your oldest data within 264 days or it will be
overwritten by newer data. The example given is conservative -- if your data does not
need the maximum storage you will get longer.
If you have multiple LOG() commands at different time intervals, the calculation
becomes more complex, but is essentially the same procedure. Any LOG()
commands that are in slow timers compared to the fast timer do not contribute much
to the overall storage and can often be ignored.