Allen-Bradley controllogix 1756-L61, Application Book

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Publication LOGIX-AP007B-EN-P - January 2007
12 CompactFlash File System Restrictions
Minimum File Size/Resolution
The minimum file size supported is 450 bytes. If you create a file and write 4
bytes to the file the application will actually write 450 bytes. The same
concepts applies for reads and writes. If you read or write 500 bytes, 900 bytes
will be read or written.
Memory Resources and Performance
The subroutines described in this document depend on messaging to perform
the required operations to the CompactFlash card. These operations use the
same memory resources that might be needed to perform other messaging.
The performance of the CompactFlash card operations will be affected by:
•
the communications system overhead time slice and amount of other
messaging or HMI communications that are occurring in the controller.
You can configure the System Overhead Time Slice from the Controller
Properties dialog.
•
the task structure of the program. Motion, periodic tasks, or event tasks
slow performance when accessing the CompactFlash card.
Refer to Appendix B, Performance Data for more information.
Controllers Supported
Only the Logix series of controllers support read/write access to the
CompactFlash card using this feature.
Limited Life
Due to the limited life of any CompactFlash card, write to the card only when
necessary. Do not write to the CompactFlash card every program scan. Write
to the card only at predetermined intervals, such as a shift change.
Read or Write Portions of a File
The application code does not support reading or writing to portions of a file
that already exists. Only the entire file can be read or written. If a part of the
file needs to be modified, read the entire file from the CompactFlash card,
make changes, and then write the file back to the card.
Number of Files that Can Be Created
Currently, the application only supports creating files on the root directory.
Because the FAT16 format is used on the CompactFlash card you are limited
to 250 files. This is a Microsoft product limitation.