Spectrum Controls 1734sc-OE2CIH - Manual (Page 46)

4-10 Chapter 4: OE2CIH and HART
User’s Manual Pub. 0300272-01 Rev. A.0
It is considered a configuration error if the values are equal. Negative scaling is also not
allowed which means the High Value must always be greater than the Low Value.
The scaling formula is as follows:
output_mA = user * gain + offset
Where:

output_mA = The current present at the terminal block in Milliamps

user = The user value in the channels Output Word

gain = (range_high–range_low)/(user_high–user_low)
-
range_high = (20.0 for both range settings).
-
range_low = (0.0 = 0 to 20 mA range; 4.0 = 4 to 20 mA range)
-
user_high = (user-defined INT): Constrained by the above limitations
-
user_low = (user-defined INT): Constrained by the above limitations

offset = range_low–(gain*user_low)
Examples:

Range = 4 to 20 mA
-
High Value = 20000
-
Low Value = 4000
For an output value of 10000, the channel shall drive 10 mA.
It is possible to enter a value of 30000, which scales well beyond the capabilities of this
module. In this case, the output will be set to either the High Limit value (see below) or
drive to its maximum capabilities.
To achieve the highest output resolution, it is necessary to use the full 16-bit range for the
endpoints.

Range = 0 to 20 mA or 4 to 20 mA
-
High Value = 32767
-
Low Value = -32768
It is also possible to configure scaling to allow the full output limit.
The module is capable of reaching 21 mA for both ranges.

For 4 to 20 mA range, the low limit is 3 mA.

For 0 to 20 mA range, the low limit is 0 mA.
To determine what the user-defined High and Low Engineering values are, the above
formula is used to reverse scale the current to the user values. Use 21.0 as the high end
point; 0.0 or 3.0 as the low end point (depending on the range). Then scale 20.0 to the
High value and 0.0 or 3.0 to the low user value.