Schneider Electric 840 USE 106 0 manual How a 984 Hsby System Works

Theory of 984 HSBY Operation

How a 984 HSBY System Works

984 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.
Stages of State
	A Hot Standby system transfers state RAM data from the Primary to the Standby
RAM Transfer	controller while the Primary controller scans and solves the ladder logic application
	program. There are three steps in this transfer process:
	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
	The Primary CHS 110 Hot Standby module initiates the state RAM transfer
Transfer	operation. 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 transfer (controller-to-CHS 110) 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.
	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.

32 840 USE 106 00 January 2003