Campbell Hausfeld SM4M - Manual (Page 23)

Section 4. Storage Module Memory: Details of Data Storage
4-3
4.5 Bad Data
The storage module has mechanisms that detect corrupted data being received
from the datalogger. When bad data is received the module will immediately stop
storing the data. The datalogger will detect this and retransmit the entire data from
the last point that the storage module had confirmed receiving the data.
This process does have some consequence for memory use. Data is stored as
received. With flash memory it is not possible to overwrite the previously
received data transmission without erasing a complete block of memory. Instead a
marker system is used which marks the previously written data as bad. When the
data is collected, the ‘bad’ data is skipped so a continuous stream of good data is
output. However, the amount of good data that can be stored in the module is
reduced by the size of the bad data blocks.
‘Bad’ data can also be stored if the module is unplugged while receiving data
from the datalogger or if the module resets due to an electrical transient occurring
while data is being stored.
In most cases ‘bad’ data will be rare and the reduction in storage small. The
likelihood of significant loss of available storage increases if the length of data
transmissions to the module are long, and/or the module is unplugged frequently
without taking care to wait until the end of transmission.
The areas with ‘bad’ data are freed for normal storage again either when the
module is erased or when the area is erased prior to writing new data.
The only other instances when you might encounter ‘bad’ data are:
• The start of dump pointer is manually positioned in the module prior to
requesting data, and that position is in the middle of an area of ‘bad’ data.
• An SM in ring-mode has filled the memory and wrapped around the ring and
erased the block containing the start of the ‘bad data marker’ (an area of bad
data which straddles flash blocks). The next block containing the remainder
of the bad data will not have been erased.
In both cases the ‘bad’ data will be read out as normal data and written to file.
However, when the ‘end of bad data marker’ is encountered, it will be treated as if
it is a file mark and the data that follows will be written into a new file. You will
then find a small file usually containing a partial duplicate of data that is also
written to the following larger file.