Flash Disk for AC500 V2 Products

Some AC500 CPUs (e.g. PM592-ETH, PM595) are equipped with an non-removable and non-volatile flash disk. It contains no moving parts. The flash disk is designed for operation in industrial environments and applications. A wear levelling algorithm and a power-fail protected file system provide industrial robustness. To write on the flash disk the CAA_File_Library: CAA_File Library

Attention

:strong:The flash disk has a finite number \ of write cycles.

A warning will be generated when the flash disk reaches 80 % of its max. number of write cycles. The flash disk will be set into read-only mode to avoid data loss, when the max. number of write cycles is reached.

Number of max. Write Cycles

Technically, the flash chip used in flash disk has 100000 Erase-Cycles (Write cycles).

Due to the produced write overhead, the optimum achievable number of write cycles is 50000 (for typical payload sizes of 256 kB).

The write overhead is indicated by the write amplification factor (WAF).

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Rule of thumb for assessing the flash lifetime for an application:
Typical payload sizes WAF Max. write cycles
256 kB 2 50000
128 kB 4 25000
64 kB 8 12500
… … …
1024 Byte 512 < 200
512 Byte 1024 < 100

Note

Lifetime of flash disk will also depend on the operating environment.

E.g. high ambient temperatures will impose stress on the flash memory and reduce the total overwrites achievable.

Read cycles are unlimited.

Important hints to PLC programmers:

  • Programmer should keep the amount of cyclic written data low to assure long availability.
  • To increase the number of write cycles, the programmer could choose to use only 90 %, 75 % or even 50 % of the flash disk capacity. The disk space left free will automatically be used by the wear levelling algorithm.
  • The wear levelling information of the flash disk can be read from the PLC browser with the command diskcfg     settings flash disk. In applications with high data generating processes (typical more then 1,5 GByte/day) this information should be monitored during implementation.