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Using a Quantum IEC Hot Standby System
142
840 USE 106 00 January 2003
IEC application
data
To maintain consistency of the IEC application’s data between the Primary and
Standby controllers the IEC heap is transferred through a reserved area in the 3x-
register range, the so called IEC HSBY Registers. The size of this reserved area is
assigned in the PLC Memory Partition dialog (refer to
Additional Guidelines for IEC
Hot Standby , p. 147
). The size of the IEC HSBY Registers can never be smaller
than the size of the IEC heap (application data), otherwise the copy-and-transfer
mechanism does not work.
The size of the configured state RAM has a significant impact on a Hot Standby
system’s scan time: The more memory (state RAM) that is transferred on every
scan, the slower the scan (for details refer to
Theory of IEC HSBY Operation, p. 43
).
If future modifications to the IEC application are expected to be small, the safety
buffer can be correspondingly less, reducing the general memory transfer size. The
term "future modification" focuses on changes to the system that do not need the
Primary controller to be stopped, which is a "download change".
You should try to reduce the size of configured 3x-Registers for IEC usage by
adjusting it to what’s really used in terms of your particular needs regarding future
modifications. That’s why the term "safety buffer" is used with IEC Hot Standby. The
diagram above illustrates that the unused parts of the program data and DFB
instance data areas make up the safety buffer. The important thing is that the size
of the safety buffer is a configuration item, therefore it cannot change without
shutting down the system, just as with any other configuration change.
Memory
Prediction Dialog
To help optimize the size of the safety buffer and therefore the total amount of IEC
HSBY Registers to be transferred, use the Memory Prediction dialog to determine
an appropriate final configuration. This optimization with Concept 2.5 can be done
offline.
The Memory Prediction dialog shows in the Hot Standby Memory section the
numbers of bytes configured and used. To determine the number of 3X registers,
divide the number of bytes by two. As shown below, there are 10000 IEC HSBY
registers configured and 78.3% of them are used. There is, therefore, a safety buffer
of approximately 22% of the registers to allow for future application changes. After
making changes to the IEC HSBY registers in the configuration, reinvoke the
Memory Prediction dialog to view the effect on the Hot Standby Memory.