Schneider Electric 840 USE 106 0 manual System Scan Time, Primary Rack Scan, Solve Segment Scan

Theory of 984 HSBY Operation

System Scan Time

Effect on System When the ladder logic program being executed by the primary controller is longer

Scan Timethan 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.

Primary Rack		1 Scan
PLC	Solve All Segments
	PLC-to-CHS 110 State RAM
	Transfer (over the Quantum Backplane)

CHS 110

CHS 110-to-CHS 110 State RAM

Transfer (Over Fiber Optic HSBY Link)

Standby Rack

CHS 110

CHS 110-to-PLC State RAM Transfer

PLC

Solve Segment 1 Solve Segment 1

1 Scan

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

lcontaining 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

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