Schneider Electric 840 USE 106 0 manual 142

Using a Quantum IEC Hot Standby System

IEC application To maintain consistency of the IEC application’s data between the Primary and

dataStandby 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.

MemoryTo help optimize the size of the safety buffer and therefore the total amount of IEC Prediction Dialog 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.

142 840 USE 106 00 January 2003