Rockwell Automation 1746-FIO4V SLC 500 Fast Analog I/O/ User Manual - Manual (Page 49)

Publication 1746-UM009B-EN-P - September 2007
Write Ladder Logic 49
Scale Offsets When
>32,767 or <32,768 Some applications may produce an offset greater than 32,767 or less
than –32,768, the largest value that can be stored in a 16-bit integer or
processed by an SLC processor. If so, you may reduce the magnitude
of the offset by shifting the linear relationship along the input value
axis. When you compute linear relationships, you will see how the
offset is reduced in this manner. The following example applies to a
0.5…9.5V dc output scaled from a narrow input range of 90…100%.
1. First we compute linear relationships and observe that the offset
is beyond –32,768.
Use the following equations to compute linear relationships:
Notice the offset is beyond –32,768.
9.5 V = 3890
(scaled max)
0.5 V = 205
(scaled min)
Scaled
Value
Input Value
(from data table) 100%
(input max) 90%
(input min)
Scaled value = (input value x slope) + offset
Slope = (scaled max – scaled min) / (input max – input min)
Offset = scaled min – (input min x slope)
(3890 – 205) / (100 – 90) = 3685/10 369 (> 3.2767 so you cannot use SCL)
~
–
205 – [90 x (368.5)] = 205 – 33165 = –32,960
Scaled value = (input value) x (368.5) – 32,960