Schneider Electric 840 USE 106 0 manual Layout, Transferred RAM, Standby’s IEC data consistency

Theory of IEC HSBY Operation

Layout of completely transferred state RAM in an IEC Hot Standby system

Layout of	The diagram below illustrates that a significant piece of the controller’s state RAM is
transferred RAM	taken as a transfer buffer for copying the IEC heap from the Primary to the Standby
	controller. The transfer header is located at the very top of the transfer buffer. The
	transfer header contains information about the Primary’s exec version, time
	synchronization information and the IEC application’s version. This information
	allows the Standby controller, once it received the transfer buffer, to decide whether
	to remain online or go offline. When online, the Standby controller copies the
	Primary’s IEC heap out of the transfer buffer into its internal memory, which ensures
	the Standby’s IEC data consistency.

State RAM (Compl. xferred)

			Header	Program Data
0x			(Exec Vers.,
				Used
			Timing Info, ..,)
Total
				Prog. Data
				Configured
1x				Program Data
Total
				Unused
	IECforBufferHeap		heap
3xTotal			bigasSpaceas IEC	DFB Instance
				Data
		No. 3x regs
		Configured
		for IEC HSBY
	Transfer
4x				Free Memory
Total
				for addtl DFB
				Instance Data

Safety Buffer for Future

changes/additions

840 USE 106 00 January 2003 53