Multi-Tech Systems 7500 - Manual (Page 228)

Setting Analog Zero and Full Scale Values ION 7500 / ION 7600 User’s Guide
Page 228 Digital and Analog I/O Technical Note
Considerations when scaling
1. Set the Full Scale value that you expect. For example, consider scaling an
Analog Output module whose Source input is linked to kW values; if the
highest Watt value that you expect is 10 kW enter 10,000 as the Full Scale value.
2. Consider the output type you have, (-1 to 1 mA, -20 to 20 mA, 0 to 20 mA) and
then consider the output range you want (0 to 1 mA, 0 to 20 mA or 4 to 20 mA).
For example, the Analog output on an I/O Expander may be 0 to 20 mA, and the
analog device connected to the I/O Expander may support a range of 4 to 20 mA.
3. Calculate the Zero Scale setting according to the table below. The table assumes
the Source input varies between Zero Scale and the Full Scale settings.
* To determine the Zero Scale multiplier, calculate the ratio of minimum desired
output to the desired range:
For example, assume output type is 0 to 20 mA and desired range is 4 to 20 mA:
N OTE
The Zero Scale setting is minus 0.25 times the Full Scale value as explained in the following section
“Determining the Zero Scale setting for output ranges below zero”. Although the section describes setting
for ranges below zero, the calculation proves the multiplier is negative for output ranges starting at zero.
Output Type Desired Range Zero Scale Setting*
Full Scale
Setting
-1 to 1 mA 0 to 1 mA negative value of Full Scale (e.g. -1000) e.g. 1000
-20 to
20 mA
0 to 20 mA negative value of Full Scale (e.g. -1000) e.g. 1000
4 to 20 mA
-1.5 times value of Full Scale (e.g. -
1500)
e.g. 1000
0 to 20 mA 4 to 20 mA
-0.25 times value of Full Scale (e.g. -
250)
e.g. 1000
ZeroScaleMultiplier
MinimumDesiredOutput
MaximumDesiredOutput MinimumDesiredOutput –
------------------------------------------------------------------------------------------------------------------------------------------- =
0.25
4mA
20mA 4mA –
---------------------------------- =