Spectrum Controls 1746sc-INI4vi - Manual (Page 38)

34 SLC 500 ™ Isolated Analog Input Modules
The following equations show you how to convert user-defined scale
units (or any type of units) to engineering units, and vice versa:
S = {(U - U
low
)
×
(
∆
S)
÷
(
∆
U)} + S
low
D = {(S - S
low
)
×
(
∆
U)
÷
(
∆
S)} + U
low
where S = signal value (in engineering units, such as psi)
S
low
= low limit of signal value
S
high
= high limit of signal value
∆
S = S
high
- S
low
D = data value (user-defined scale)
U
low
= low limit of user-defined scale
U
high
= high limit of user-defined scale
∆
U = U
high
- U
low
Example – Suppose you have a sensor with a 4–20 mA range, and you
want to scale your data from 100 to 9999 counts. For a 4–20 mA input
with user-defined scaling, your module sets the signal limits to 3.5 mA
and 20.5 mA (see Table 14). After entering 100 and 9999 into output
words 4 and 5 (or 6 and 7), respectively, the relationship between input
signal and data value (counts) would be as follows:
Figure 10. Graph of signal value vs data value
Data Value
9999
100
20.5 mA 3.5 mA
Signal
Value