The DeltaV system uses unique Device Signal Tags (DSTs) to provide a contextual meaning to device signals. A default DST, based on the signal specification, is created for each channel. For CHARM I/O, DSTs are derived from the CIOC name and the CHARM address in the form: CIOCNameCHM1-02. DSTs are used to link a field signal to the control strategy and to provide flexibility during control strategy development by allowing the physical I/O to be defined late in the project. This keeps all control strategy references valid when a channel is assigned to a new controller.
Typically, users browse for DSTs from Control Studio when they are assigning a function block to an I/O signal. Like other I/O, CHARM DSTs are browsed with the Browse dialog. There are different types of device tags available to the browser: valid I/O references, unassigned I/O references, and All. When browsing for CHARM DSTs keep the following in mind about the different types of DSTs presented by the browser:
Valid I/O references: Presents a list of defined DST references that are suitable for the selected function block. This list is presented in context with controllers. Assign DSTs to the same controller as the control module.
Unassigned I/O references: Presents a list of DSTs that are not bound to a physical channel. A signal type is not verified until the DST is bound to a physical channel.
All: Presents a list of all valid and unassigned I/O references. This list includes all the DSTs defined in the system.
A DST differs from a SCADA tag.
In addition to I/O function blocks, I/O references can be created using external parameter references. The browser can be used to define external parameter references and the reference can be made to module parameters, I/O parameters, diagnostic parameters, or to DST parameters:
Module parameters: References are made to module parameter paths starting with the module name. Most runtime parameters in the system can be referenced.
I/O parameters: References are made to physical I/O parameter paths starting with the controller name.
To ensure that CIOCs provide the highest possible I/O performance, external and dynamic references to diagnostic parameters of CIOCs are not reported in modules downloaded to controllers and do not resolve at run time. If you need to use these types of parameter references in your control strategy download the modules to a workstation's virtual controller.
DST parameters: Refer to the preceding descriptions of Valid I/O references, Unassigned I/O references, and All.