St Modules Vs Hs Modules - Siemens SIMATIC ET 200AL System Manual

8.2

ST modules vs HS modules

This section gives you an overview of other important distinctions between standard and
high-speed analog modules. The specified numerical values are examples of selected
modules. Keep in mind that the information can vary from module to module. The exact
information of specific used analog module is available in the corresponding manual.
Conversion method
Instantaneous value conversion
Instantaneous value conversion if often used to convert analog signal into digital information
in HS modules. In this type of conversion a very short sample is taken from the measured
signal and converted into digital data. This digital information therefore represents an
instantaneous value of the measured signal. Instantaneous value conversion is used to
acquire fast changes of process values, such as in the area of motion control. The quickly
changing physical quantities in this case must be acquired and processed quickly in
cooperation with suitable transducers. Input areas are primarily voltage, current and
resistance, the latter especially for position potentiometers.
Because the measured signal is converted within a short time, there is a risk of incorrect
measurements in case of faulty signals. The analog inputs working with the instantaneous
value conversion method cannot detect if the sampled signal value is the target signal or an
overlaying disturbing pulse. They always convert the "instantaneously" detected value. This
means the pending measurement task should be carefully examined with regard to time
constraints.
Integrating conversion
ST modules usually work according to the integrating conversion principle. The integrating
conversion involves the sampling of a measured signal across a specified time period
(integration time). Although this method is slower than instantaneous value conversion, it
returns a conversion with higher immunity to interference. When you select the time interval
for the conversion so that it corresponds to one or several periods of the surrounding
network frequency, the omnipresent interference that is possibly also overlaying the signal is
basically suppressed. SIMATIC modules give you the option to set the network frequency at
which the plant is operated by means of parameter assignment. The resolution and
conversion times may change depending on the interference frequency suppression that is
set. The technical specifications of the respective module define these dependencies.
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
High-speed analog modules
8.2 ST modules vs HS modules
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