Rockwell Automation 57C423 Common Memory Module - Manual (Page 18)

4Ć2
4.1.2 Variable Storage
Local variables are stored on the Processor module in which the
application task that declares them will run. Common variables are
stored on the Processor module, unless there is a Common Memory
module in slot 0 of the rack, in which case the common variables are
stored on the Common Memory module instead. This frees up
memory on the Process module for application tasks. Because
common variables can be stored on either the Processor module or a
Common Memory module if it is located in slot 0, a variable defined
in the configuration and declared common in an application task is
not necessarily one that is stored on the Common Memory module.
In other words, when used to describe a variable in AutoMax,
common" refers strictly to variable control, not to variable storage.
A Common Memory module located in a nonĆ0 slot is treated by the
system as two generic" I/O modules with 32K registers each (32K in
the actual slot location and 32K in the slot to the immediate right).
Because all I/O variables are considered common by the system,
variables defined on this module are treated like any other common
variables for storage purposes: they are stored on the Processor
module in a singleĆProcessor rack, or on the Common Memory
module in slot 0 if it is installed.
When the Common Memory module is in slot 0, the allocation of its
memory is under system control. During configuration, the user
specifies only the name and type (e.g., boolean)of each common
variable. These variables are accessible to application tasks and
Programming Executive functions (e.g., monitoring)by name only.
If the Common Memory module is in any other even slot, memory on
the module is allocated by register/bit address exactly as specified by
the user during configuration. These variables are accessible to
application tasks and Programming Executive functions by name or
register/bit address
4.1.3 Configuration Method
The method used to configure the Common Memory module
depends both upon the Programming Executive software version
being used, and on the slot number (0 or any other even slot)of the
module. For Programming Executive software V3.0 and later, the
module is configured like all other modules, i.e., using the Rack
Configurator menu option. See JĆ3750, the Programming Executive
V3.0 instruction manual for information on configuring the module in
both slot 0 and other even slots.
For Programming Executive software V2.1 and earlier, the Common
Memory module is configured in a special type of application task
called the configuration task, one of which is required per rack. See
Appendix C for detailed instructions on configuring the module if you
are using V2.1 and earlier of the Programming Executive software.
4.2 Restrictions
This section describes limitations and restrictions on the use of this
module.