Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.2 Precision/resolution
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
15
Over and underrange
It may happen for control processes with high signal jumps that the settling curve of the
signal briefly leaves the rated range until it reaches the setpoint. The over and underrange
ensures that no error is reported while the signal is not in the rated range. If the signal
exceeds the over or underrange, however, and reaches the overflow or underflow range, the
error state "Overflow" or "Underflow" is diagnosed. This means the over or underrange
correspond to a tolerance range between the rated range and the overflow or underflow.
Additional information on the settling time is available in the section Settling and response
times of analog output modules (Page 52).
The figure below shows the rated range, the overrange and overflow of a unipolar measuring
range. The signal leaves the rated range briefly during the settling phase.
①
Analog output signal
t
E
Settling time of the signal to the setpoint
t
1
Module ends conversion at the terminal of the analog output channel and outputs the signal
t
2
Signal has settled and the specified analog output value has been reached
Analog module precision
The precision of an analog module is specified as a percentage value or absolute value, for
example, in K or °C. This characterizes the total error of measured value acquisition. The
basic error limit at 25 °C, as well as the operational limit, are specified in accordance with
international standard IEC 61131, which also forms the basis for European Standard
EN 61131. For more information on the operational/basic error limits, refer to the Operational
and basic error limits (Page 23) section.