Accessories • CompactFlash cards
Chapter 6 Accessories
Panel PC 800 User's Manual V1.30
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9 CompactFlash cards
9.1 General information
CompactFlash cards are storage media that are easy to replace. Due to their robustness against environmental
influences (e.g. temperature, shock, vibration, etc.), CompactFlash cards are ideal for use as storage media in
industrial environments.
9.2 General information
In order to be suited for use in industrial automation, CompactFlash cards must be highly reliable. The following
items are very important to achieving the necessary level of reliability:
•
The flash technology used
•
An efficient algorithm for maximizing service life
•
Good mechanisms for detecting and fixing errors in the flash memory
9.2.1 Flash technology
Currently, CompactFlash cards are available with MLC (multi-level cell) and SLC (single-level cell) flash blocks.
SLC flash memory has a service life 10 times longer than MLC, which is why only CompactFlash cards with SLC
flash blocks are suited for industrial applications.
9.2.2 Wear leveling
Wear leveling is an algorithm that can be used to maximize the service life of a CompactFlash card. There are
•
No wear leveling
•
Dynamic wear leveling
•
Static wear leveling
The basic idea behind wear leveling is to distribute data over a broad area of blocks or cells on the disk so that the
same areas don't have to be cleared and reprogrammed over and over again.
9.2.2.1 No wear leveling
The earliest CompactFlash cards didn't have an algorithm for maximizing service life. The service life of a Com-
pactFlash card was determined only by the guaranteed lifespan of the flash blocks.
9.2.2.2 Dynamic wear leveling
Dynamic wear leveling makes it possible to utilize unused flash blocks when writing to a file.
If the disk is 80% full with files, then only 20% can be used for wear leveling.
The service life of the CompactFlash card is therefore dependent on the amount of unused flash blocks.
9.2.2.3 Static wear leveling
Static wear leveling monitors which data is rarely modified. From time to time, the controller then moves this data
to blocks that have already been used frequently in order to prevent further wear on those cells.
9.2.3 ECC error correction
Bit errors can be caused by inactivity or when a certain cell is being operated. Error correction coding (ECC)
implemented via hardware or software can detect and correct many errors of this type.
9.2.4 S.M.A.R.T. support
Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.) is an industry standard for mass storage devices
that has been introduced to monitor important parameters and quickly detect imminent failures. Critical performance
and calibration data is monitored and stored in order to help predict the probability of errors.
