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Additional Guidelines for IEC Hot Standby
152
840 USE 106 00 January 2003
Data Memory,
Continued
This value alone is not enough to verify whether or not the application fits, since we
need 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.