Spectrum Controls 1746sc-INO4vi - Manual (Page 33)

Chapter 4: Using Your Output Module 23
User-Defined Scale
For special applications, the 1746sc-INO4i and 1746sc-INO4vi output
modules let you define a custom data format. This “user-defined scale” is
very similar to the “proportional counts” data format—except that instead
of converting the output data to an output signal using a previously
defined scale (-32,768 to 32,767), your module converts the output data
using a scale defined by the values in output words 6 (low limit of scale)
and 7 (high limit of scale).
The high limit value must be greater than the low limit value for proper
operation. Also, the difference between the low and high values should
be greater than 1024 counts. If the difference between the low and high
values is less than 1024 counts, unexpected results can occur (especially
at the extreme ends of the range).
You select the data format for each channel using that channel’s
configuration bits, described in the previous subsection, Configuring
Each Output Channel.
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 value of user-defined scale
U
high
= high value of user-defined scale
∆
U = U
high
- U
low
Example – Suppose you have a valve with a 4–20 mA range, and you
want your scale to go from 100 to 9999 counts. For a 4–20 mA output
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 6 and 7, respectively, the relationship between data value (counts)
and output signal would be as follows: