Rockwell Automation 2706-MB1 MESSAGEVIEW 421 SOFTWARE MANUAL - Manual (Page 88)

4–40 Planning an Application
Publication 2706-817
Data Limits for a scaled variable
If a value exceeds the range for the selected data type, the terminal
will display a “Value Not in Range” warning. When using scaling, it
is a good idea to limit the minimum and maximum values an
operator can enter. These values are set in the Tag Editor Form View.
The range for scaled values should be calculated before the Tag
Form dialog is filled out. Use the following formula to determine
scaled minimum and maximum values:
Maximum Input Value =
Minimum Input Value =
Maximum Value for Data Type x Scale + Offset
Minimum Value for Data Type x Scale + Offset
Example:
Scale = 1.8 and Offset = 32 (Fahrenheit to Celsius conversion)
Data type is Signed Integer (-32,768 to +32,767)
The minimum and maximum values an operator can enter without
truncating:
Maximum Input Value =
Minimum Input Value =
32,767 x 1.8
-32,768 x 1.8
+ 32 = 59012
+ 32 = -58950
Initial value of a scaled variable
The initial value specified in the Tag Form dialog is written to the
controller upon powerup or reset. This initial value is not scaled, so it
must be entered in the units used by the logic controller.
For instance, assume the terminal uses
°
C and the controller uses
°
F.
To initialize the terminal to a value of 32
°
F, the initial value in the
Tag Editor needs to be 32, not 0.