YOKOGAWA STARDOM FCN-500, Engineering Manual

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<2. Basic Design and Function Design>
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TI 34P02K35-02E Jun. 6, 2018-00
● Behavior of Retain Data on Online Download
Even if a modification involves a change in the retain data area, persistency of
retain data is guaranteed so long as the modification is online downloaded. Newly
added retentive variables, however, will be set to initial values.
● Relationship between Initial Value and Retained Value
After an FCN-500 or FCN-RTU warm start, even if initial values are specified for
retain dat, their retained values take precedence.
● Summary
By default setting, FCN-500 or FCN-RTU is configured so that retain data is
resident in non-volatile memory. As such, you can first review the specification on
this basis.
Data resident in non-volatile memory retain their values even after the FCN-500
or FCN-RTU is powered off because of a backup battery. These retained values
are restored at the next FCN-500 or FCN-RTU power on. In this way, the most up-
to-date data values are always maintained so keeping retain data resident in non-
volatile memory is the usual practice.
However, even if retain data is made resident in non-volatile memory, we
recommend saving retain data to the flash memory regularly as a safeguard
against unexpected situations where data retained in memory cannot be used. In
addition to saving retain data manually, saving data regularly using a control
application is also recommended.
SEE ALSO
For details, see Section 9.1.12, “Logic for Saving Retain Data” of Chapter 9, “Advanced
Engineering.”
If a backup of retain data is saved on the flash memory, in case of event that data
retained in non-volatile memory cannot be used for whatever reason, retained
data values as at the time of saving will be restored from the system card. by
doing so, it avoids the worst-case scenario where all retain data variables are
initialized.
Compared to keeping retain data resident in non-volatile memory, keeping retain
data reside in volatile memory enables a shorter execution time, and hence a
shorter scan cycle for FCN-500 or FCN-RTU operation. However, if the FCN-500
or FCN-RTU is powered off and on again, retain data values before power off is
lost and retain data values are always restored from the flash memory.
To prepare for unexpected contigency, save retain data to the flash memory
regularly, and also by manually whenever retain data is modified.
SEE ALSO
For details on scan cycle, see Section 2.2.4, “Determining FCN-500 and FCN-RTU Scan Cycle.”