RSView Supervisory Edition Installation Guide

10 Creating derived tags
This chapter describes:
• what derived tags are and how to use them.
• setting up derived tags.
• using the Derived Tags editor.
• creating a derived tags component.
• what the maximum update rate is.
• starting and stopping derived tag processing.
About derived tags
A derived tag is a tag whose value is the result of an expression. An
expression can be simply a tag name, or it can be an equation made up
of tag names, mathematical and logical operations, special functions,
and If-Then-Else logic. Derived tag calculations are performed on the
HMI server.
A derived tags component is a file that contains the definitions of one
or more derived tags. A derived tags component can be run when an
HMI server starts, or it can be run using the DerivedOn command,
after the HMI server has started.
Writing to tags whose value is the result of a derived tag expression is
not recommended because a derived tag is evaluated when the tags in
the expression change. For example, if the derived tag called Tag3 has
the expression of Tag1+Tag2, and if you set Tag3=0, the value of Tag3
becomes zero, and will also be updated when the value of Tag1 or Tag2
changes.
Creating derived tags n 10–1