Rockwell Automation 57C423 Common Memory Module - Manual (Page 17)

4Ć1
4.0 PROGRAMMING
This section describes how the data is organized in the module and
provides examples of how the module is accessed by the application
software.
The Common Memory module has two distinct modes of operation
depending on the slot that it is in. If the module is located in slot 0,
the bus arbitration logic and watchdog timer are enabled and the
module is controlled entirely by the leftĆmost Processor module. If it is
in any other even slot, the bus arbitration logic and watchdog timer
are disabled and the module provides userĆconfigurable data storage
only. This mode is useful if you need to control explicitly the physical
allocation of memory, e.g., to define consecutive registers for shift
register instructions. Note, however, that it is not possible to define
arrays in this mode.
4.1 Configuration
Before creating AutoMax application tasks, the user must configure
the hardware in the system. Configuration is the process of assigning
variable names to I/O and memory locations in modules located in
the rack. The configuration process makes it possible to create
application tasks that reference variable names instead of fixed
locations. This information must be loaded onto the Processor
modules in the rack before application tasks can run.
This section outlines some basic system parameters that determine
how to configure the Common Memory module and reference
memory stored on the module. Unless specifically noted otherwise,
the information below applies to both M/N 57C413B and M/N 57C423
Common Memory modules as they operate in both DCS 5000 and
AutoMax systems. Recall that when either module is in an even slot
other than 0, only 64K x 16 worth of memory (two logical slots in the
rack)is available for configuration by the user.
4.1.1 Variable Control and Access
To understand the effect of a Common Memory module in a rack, it is
important to understand variable control in AutoMax systems. There
are two types of variable control in AutoMax systems: common and
local. Control" refers to whether the variable will be accessed
exclusively by one application task (local), or whether it must be
accessible for reads or writes by more than one application task in
the rack (common). Defining variable control is accomplished as
follows:
local variable: a)default control type; not defined
in configuration
common variable: a)defined in configuration; there are
two types, memory and I/O. If
memory, can be volatile or
nonĆvolatile
Variable access in an application task is enabled by declaring the
variable common or local in application tasks that reference the
variable. See the appropriate programming language reference
manuals for specific information on declaring variables local or
common.