Rockwell Automation 9301 Series RSView32 Users Guide - Manual (Page 119)

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5–1
Chapter 5
Creating derived tags
A derived tag is a tag whose value is the result of an expression. The
expression can be made up of mathematical operations, tag values
from the value table, if–then–else logic, and other special functions.
You should not write to derived tags, because the derived tag is only
evaluated when the expression changes. For example, if the derived tag
tag3 has the expression of tag1 + tag2, and if you set tag3 = 0, the
value of tag3 becomes zero, and will only be updated when the value
of tag2 or tag3 changes.
The value table is the part of memory that stores current tag values
when RSView32
™ is running. The current value of the derived tag is
stored in an analog, digital, or string tag in the value table. Derived tags
can be device tags or memory tags.
How to use derived tags
Here is an example of how a derived tag can be used. Suppose there
are five weight sensors on a conveyor belt. The tag database contains
one tag for each sensor, so the weight at each point on the conveyor
belt is monitored. If the weight at any point is excessive, RSView32
triggers an alarm.
However, what happens if no individual sensor detects an excessive
weight, but the total of all five sensors is too high? In this case, you
could set up a derived tag to sum the weights of all five sensors and
store the result in the value table. Then, if this total is too high,
RSView32 can trigger an alarm.