Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.4 Unscaling analog values
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
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2.4 Unscaling analog values
Unscaling
The output of scaled analog values often requires conversion of the analog value calculated
by the user program into the value range of the analog output module. This conversion is
referred to as unscaling.
Unscaling blocks
STEP 7 offers the suitable program block for unscaling of analog values. The UNSCALE
block is included in the STEP 7 scope of delivery and permits the input of a high and low limit
that defines the range of the program value.
Example
An analog value from 0.0 to 100.0% calculated by the user program is to be converted with
the UNSCALE block to the value range -27648 to +27648. When the unscaled value is
output to an analog output module, this module addresses an analog actuator (e.g.,
modulating valve). The actuator is to be addressed at a program value of 0% with the
minimum value (-10 V or -20 mA) and at 100% with the maximum value (+10 V or +20 mA).
Figure 2-8 Unscaling in the UNSCALE program block
The value calculated by the program must be transferred in the REAL format (IN). You use
the input LO_LIM to determine the low limit (0.0%), and HI_LIM to determine the high limit
(100.0%) within which the program value moves. The unscaled value is output in INT format
at the OUT output. You can assign parameters via the input BIPOLAR to specify if only
positive or only negative values are to be converted. If the parameter receives an operand
with the state '0', there is a unipolar scaling. An operand with the state '1' results in a bipolar
scaling. The output RET_VAL outputs an error code in case of an error and the value '0' in
case of an error-free execution.
Additional information on the UNSCALE block can be found in the STEP 7 online help.