Testing the user program
7.3 Analog value representation
SM331; AI 8x12 Bit Getting Started part 1: 4 -20mA
7-8
Getting Started, 11/2006, A5E00253410-02
7.3
Analog value representation
Proceed as follows
The analog values are only processed by the CPU in binary form. Analog input modules
convert the analog process signal into a digital format (16 bit word).
Five ranges have to be taken into account when converting from digital to analog values:
Representation of analog values in current measuring ranges 4 to 20 mA
Hex value
Current range
Comment
Meaning
7FFF
22.96 mA
7F00
Overflow
From hex value 16#F700 on, the sensor
value is above the configured measurement
value range and is no more valid.
7EFF
22.81 mA
6C01
Overload range
This range corresponds to a tolerance band
before the overflow range is reached. Within
this range the resolution is not optimal
though.
6C00
20 mA
5100
15 mA
1
4 mA + 578.7 nA
0
4 mA
Rated range
The nominal range is the normal range for
recording measurement values. This range
guarantees optimal resolution.
FFFF
ED00
1.185 mA
Underload range Range corresponding to the overload range
but for low values.
ECFF
8000
Underflow
From hex value 16#ECFF on, the sensor
value is below the configured measurement
value range and is no more valid.
It is necessary to convert the binary format of the values in order to display analog process
values. In our example, mA are displayed. This is done by converting the display of analog
values in mA in a programmed function (FC1).
Note
In our example, we look at the values from the output of the transducer.
Using a amperemeter, you can now compare the values on the meter with the values of the
analog values display. The values will be identical.
