Rockwell Automation 2711E PanelBuilder 1400e Modbus Communications - Manual (Page 41)

6–4 Defining Tags
Publication 2711E-6.12 – August 1997
Scale and Offset
Type a number. For the scale, do not use 0.
Scale and offset together transform raw controller data into
meaningful units for the operator. At runtime, all data read from
the controller into an analog tag will be scaled using the formula:
y = (mx) + b
where
m = scale
x = raw PLC data
b = offset
y = scaled value
The terminal displays the scaled value. When data is written to
the controller using an analog tag, the value will be inversely
scaled using the formula:
x +
ǒy * bǓ
m
This converts the value back to raw controller data before it is
written to the PLC. With Scale = 1 and Offset = 0 (the default
values for a new analog tag), the controller data is not changed.
Important: A fractional scale or offset value will result in a
floating point scaled value. This will be true
regardless of the tag’s data type. Extensive use
of floating point values can slow a terminal’s
performance.
Important: If the descaled value to be written to the tag
address is a fraction, but the tag is not
configured to deal with floating point data (i.e.,
Float data type or Default data type with a tag
address in the Float section of the controller),
the value will be truncated to the nearest whole
number before it is written to the tag address.
Note: You can also use expressions to perform calculations on
the values of analog and digital tags that read information from
the controller. See Chapter 7, “Creating Expressions,” in the
PanelBuilder 1400e Configuration Software for Windows User
Manual.
Units
This field is reserved for RSView databases. You cannot enter
anything in the Units field.
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