Schneider Electric 840 USE 106 0 How an IEC Hsby System Works, IEC Theory, State RAM Defined

Theory of IEC HSBY Operation

How an IEC HSBY System Works

IEC Theory	Both the Primary and the Standby backplanes contain a CHS 110 Hot Standby
	module. The modules monitor their own controller CPU and communicate with each
	other via fiber link. The Primary controller keeps the Standby informed of the current
	state of the application by transferring state RAM values to the Standby controller
	during every logic scan. RIO head communications are also verified.
State RAM
	A Hot Standby system transfers state RAM data from the Primary to the Standby
Transfer	controller while the Primary controller scans and solves the IEC logic application
	program. There are three steps in the transfer process:
	Stage	Description
	1	Primary controller-to-Primary CHS 110 state RAM transfer.
	2	Primary CHS 110-to-Standby CHS 110 state RAM transfer.
	3	Standby CHS 110-to-Standby controller state RAM transfer.
State RAM
Defined
	Note: Schneider Electric defines State RAM as RAM memory that is used to hold
	register and discrete inputs and outputs and internal data storage. State RAM is
	allocated to the four different reference types: 0xxxx, 1xxxx, 3xxxx, and 4xxxx.
State RAM
	The state RAM transfer operation is initiated by the Primary CHS 110 Hot Standby
Transfer Initiated	module. The module requests specified state RAM information from the Primary
	controller.

At the beginning of each scan, the Primary controller transfers the current state RAM data to the CHS 110 Hot Standby module.

As soon as the controller-to-CHS 110 transfer finishes, the Primary controller resumes scanning user logic and servicing I/O. The state RAM data is simultaneously transferred from the Primary CHS 110 module to the Standby CHS 110 module over the fiber optic link at a rate of 10 megabaud. In turn, the Standby CHS 110 module transfers the state RAM data to the Standby controller.

46 840 USE 106 00 January 2003