Memory Card Precautions; Memory Card Recognition Time - Omron CJ2 CPU UNIT SOFTWARE User Manual

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Memory Card Precautions

Confirm the following items before using a Memory Card.
Format
Memory Cards are formatted before shipping. There is no need to format them after purchase. To for-
mat them once they have been used, always do so in the CPU Unit using the CX-Programmer. If a
Memory Card is formatted directly in a notebook computer or other computer, the CPU Unit may not
recognize the Memory Card. If this occurs, you will not be able to use the Memory Card even if it is
reformatted in the CPU Unit.
Number of Files in Root Directory
There is a limit to the number of files that can be placed in the root directory of a Memory Card (just as
there is a limit for a hard disk). Although the limit depends on the type and format of the Memory Card,
it will be between 128 and 512 files. When using applications that write log files or other files at a spe-
cific interval, write the files to a subdirectory rather than to the root directory. Subdirectories can be cre-
ated on a computer or by using the CMND(490) instruction.
Number of Writes
Generally speaking, there is a limit to the number of write operations that can be performed for a flash
memory. For the Memory Cards, a limit of 100,000 write operations has been set for warranty purposes.
For example, if the Memory Card is written to every 10 minutes, over 100,000 write operations will be
performed within 2 years.
Minimum File Size
If many small files, such as ones containing only a few words of DM Area data, are stored on the Mem-
ory Card, it will not be possible to use the complete capacity of the Memory Card. For example, if a
Memory Card with an allocation unit size of 4,096 bytes is used, at least 4,096 bytes of memory will be
used for each file regardless of how small the file is. If you save 10 words of DM Area data to the Mem-
ory Card, 4,096 bytes of memory will be used even though the actual file size is only 68 bytes. Using
files of such a small size greatly reduces the utility rate of the Memory Card. If the allocation unit size is
reduced to increase the utility rate, however, the access speed will be reduced.
The allocation unit size of the Memory Card can be checked from a DOS prompt using CHKDSK. The
specific procedure is omitted here. Refer to general computer references for more information on allo-
cation unit sizes.

Memory Card Recognition Time

Several seconds is normally required for the CPU Unit to recognize the Memory Card after it is inserted.
The required time depends on the PLC's cycle time, the Memory Card Capacity, the number of files
stored on the Memory Card, and other factors. For example, the recognition time for an HMC-EF183
Memory Card will be roughly 8 s if the cycle time is 0.4 ms and all PLC Setup parameters are set to the
default values
CJ2 CPU Unit Software User's Manual
7 File Operations
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