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Goals and Symptoms
To explain why ithe scan time for the Quantum HSBY is 4-5 times longer than it would normally be for a
standalone Quantum plc when a SCADA is connected using an NOE771xx.
Facts and Changes
This is a normal behaviour of the Hot Standby function.
Causes and Fixes
The reason for the long scan time delay is because the IEC Hot Standby function normally takes about
four to five times the scan time of a standalone controller. A delay in the SCADA's Modbus TCP/IP
communications update rate can also result in adding to the long scan time.
The use of 'Unlocated' variables in a 'Hot Standby' plc application can also result in a long scan time. The
reason is because 'Unlocated' variables are restricted to using only one master path. However, 'Located
Varaiables' are able to use up to four master paths. It is recommend that 'Located' variables be used in the
application whenever it is possible.
The scan time could also get extended if the defined 'IEC HotStandby Data Area' is too large or under sized.
The size of the 'IEC HotStandby Data Area' should be 25 percent more than the recommended size of the data area.
If Concept is used as the PLC programming software, the 'Analyze Program' function can be used to help determine
if the data area is too small.