Schneider Electric 840 USE 106 0 manual 152

Additional Guidelines for IEC Hot Standby

Data Memory, This value alone is not enough to verify whether or not the application fits, since we

Continuedneed to know how many IEC HSBY Registers (3x) can be reserved to carry the data from the Primary to the Standby controller. The diagram below shows that 11,022 words out of 65,024 are already taken for I/O references and located variables. Therefore the maximum for IEC HSBY Registers would be 65,024 – 11,022 = 54,002 words ~ 108,000 bytes. This is more than what is actually used for application data (76,801 bytes), so that the application would fit IEC Hot Standby requirements.

The maximum size of the safety buffer for future modifications would be:

108,000 – 76,801 = 31,199 bytes which is (31,199 / 76,801) ~ 41%

Depending on how much safety buffer is required for this particular application, the final size of the IEC HSBY Registers could be determined. That, together with the table presented in Theory of IEC HSBY Operation, p. 43, would give an idea about the application’s overall scan time when operated in IEC Hot Standby mode.

152 840 USE 106 00 January 2003