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Theory of IEC HSBY Operation
840 USE 106 00 January 2003 47
System Scan Time
Effect on System
Scan Time
The effect on system scan time of any Hot Standby system depends on how much
state RAM is going to be transferred from Primary to Standby. A Hot Standby system
always has a higher scan time than a comparable standalone system.
The following has been done to provide information that allows you to forecast a Hot
Standby system’s scan time:
Calculation of overall scan time for a normal Hot Standby baseline configuration
containing minimum logic as a reference
Calculation of a PLC specific constant that expresses the increase of overall scan
time related to an increase of memory to be transferred
The normal Hot Standby configuration state RAM contains:
In the local rack: power supply (CPS), PLC (CPU), RIO Head (CRP 93x), Hot
Standby module (CHS)
In one remote IO drop equipped with 8 I/O modules, power supply (CPS) and
remote adapter (CRA)
Only the logic for the scan time evaluation