Iec 61131-3 Second edition 2003-01 Programmable controllers – Part 3: Programming languages

Figure 19 b) - Skeleton function block and program declarations for configuration example
2.7.1 Configurations, resources, and access paths
Table 49 enumerates the language features for declaration of configurations, resources, global
variables,
access paths and instance specific initializations. Partial enumeration of TASK declaration
features is also given; additional information on tasks is provided in 2.7.2. The formal syntax for these
features is given in B.1.7. Figure 20 provides examples of these features, corresponding to the
example configuration shown in figure 19 a) and the supporting declarations in figure 19 b).
The ON qualifier in the RESOURCE...ON...END_RESOURCE construction is used to specify the type of
“processing function” and its “man-machine interface” and “sensor and actuator interface” functions
upon which the resource and its associated programs and tasks are to be implemented. The
manufacturer shall supply an implementation-dependent resource library of such functions, as
illustrated in figure 3. Associated with each element in this library shall be an identifier (the resource
type name
) for use in resource declaration.
NOTE The RESOURCE...ON...END_RESOURCE construction is not required in a configuration
with a single resource. See the production single_resource_declaration in B.1.7
for the syntax to be used in this case.
IEC 2494/02
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61131-3 
 IEC:2003(E)
– 111 –
The scope of a VAR_GLOBAL declaration shall be limited to the configuration or resource in which it is
declared, with the exception that an access path can be declared to a global variable in a resource
using feature 10d in table 49.
The VAR_ACCESS...END_VAR construction provides a means of specifying variable names which
can be used for remote access by some of the communication services specified in IEC 61131-5. An
access path
associates each such variable name with a global variable, a directly represented variable
as defined in 2.4.1.1, or any input, output, or internal variable of a program or function block.
The association shall be accomplished by qualifying the name of the variable with the complete
hierarchical concatenation of instance names, beginning with the name of the resource (if any),
followed by the name of the program instance (if any), followed by the name(s) of the function block
instance(s) (if any). The name of the variable is concatenated at the end of the chain. All names in the
concatenation shall be separated by dots. If such a variable is a multi-element variable (structure or
array
), an access path can also be specified to an element of the variable.
It shall not be possible to define access paths to variables that are declared in VAR_TEMP,
VAR_EXTERNAL or VAR_IN_OUT declarations.
The direction of the access path can be specified as READ_WRITE or READ_ONLY, indicating that the
communication services can both read and modify the value of the variable in the first case, or read
but not modify the value in the second case. If no direction is specified, the default direction is
READ_ONLY.
Access to variables that are declared CONSTANT or to function block inputs that are externally
connected to other variables shall be READ_ONLY.
NOTE The effect of using READ_WRITE access to function block output variables is