Campbell Scientific SM4M/SM16M Storage Modules - Manual (Page 25)

Section 4. Storage Module Memory: Details of Data Storage
4-3
calculated by taking the Storage Module size minus 4 bytes per block (used for
internal marking purposes), MINUS the worst case erased data size.
Table 4-1. Ring-Memory Effective Memory Capacity
Module Bytes Low-Res Data Values
SM4M 4104516 2052258
SM16M 16686660 8343330
The amount of storage available for fill and stop memory is 89524 bytes or
44762 low-resolution data values greater than the above (the last eight bytes of
memory cannot be written to).
The module will only store data up to the end of the last complete block of data
transmitted before the module becomes full. This means that if data is sent in
large blocks and the module becomes full just before receipt of the end of the
last block, then the effective capacity will be the above figures minus almost
the block size.
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 areas.
“ 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 is:
•
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.