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Theory of 984 HSBY Operation
840 USE 106 00 January 2003 33
System Scan Time
Effect on System
Scan Time
When the ladder logic program being executed by the primary controller is longer
than the CHS 110-to-CHS 110 transfer, the transfer does not increase total system
scan time. However, if the ladder logic program is relatively short, the scan finishes
before the CHS 110-to-CHS 110 data transfer and the data transfer increases total
system scan time.
The following timing diagram shows how the transfer takes place.
The effect on system scan time of any Hot Standby system depends very much 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 because of the required PLC to CHS data transfer time.
Since the data transfer depends on the PLC type in the system, the following
provides 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 state RAM memory to be transferred
Primary Rack
Standby Rack
Solve All Segments
1 Scan
PLC
CHS 110
PLC-to-CHS 110 State RAM
Transfer (over the Quantum Backplane)
CHS 110-to-CHS 110 State RAM
Transfer (Over Fiber Optic HSBY Link)
CHS 110
PLC
CHS 110-to-PLC State RAM Transfer
Solve Segment 1 Solve Segment 1
1 Scan