Using a Quantum IEC Hot Standby System
126
840 USE 106 00 January 2003
Updating Project
Global Data
With a logic mismatch, project global data will be updated with every scan. Global
data that do not exist on both controllers is not updated.
The project global data that is updated includes:
All variables declared in the Variable-Editor
All Constants declared in the Variable Editor
All section and transition variables
Hot Standby behavior for project global data updating is:
All declared variable/constant will be updated every scan as long as they are
declared on both controllers
If a complete download is done to the controller that did not receive the download
change, then both controllers will have equal logic and therefore the Standby
controller gets updated fully.
If, due to a download change, a project global variable/constant has been deleted
first, and then redeclared, this variable/constant would be treated as a NEW
variable/constant, even if the same name is used. The update procedure must be
followed to bring the controllers to an equalized state.
Nontransfer Area
of State RAM
Although customizing transfers is not an option, you should designate a block of 4x
registers as the nontransfer area. These registers are ignored when state RAM
values are transferred from the Primary controller to the Standby. Placing registers
in the nontransfer area is one way to reduce scan time because the Primary PLC to
CHS transfer time is shorter. See
State RAM, p. 129
for more detail.
Note: This is true whether these variables/constants are used in the controller
program or not. Unused variables consume space and require time to be
transferred from the Primary to the Standby controller. It is not recommended to
have many variables that are defined but not used in the Primary controller
program.
