Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.14 Diagnostics
Analog value processing
42
Function Manual, 06/2014, A5E03461439-AC
Wire break in analog output modules
The analog signal output is used for wire break detection. Reliable wire break diagnostics is
not possible if the voltage is too low. In this case, the diagnostics function is deactivated
without triggering a change of the diagnostics status.
The current limit up to which the module will diagnose wire break can be found in the
technical data for the module.
Overflow/underflow
The resolution of the measuring range of an analog input module distinguishes between the
rated range, the overrange or underrange, and the overflow or underflow. The following table
assign the number of increments into which the measured signal is divided to the various
voltage measuring ranges.
Table 2- 15 Voltage measuring range ±10 V of an analog input module
Value (increments) Voltage measuring range Range
Decimal ±10 V
32767 > 11.759 V Overflow
32511 11.759 V Overrange
27649
27648 10.0 V Rated range
0 0 V
-27648 -10 V
-27649 Underrange
-32512 -11.759 V
-32768 < -11.759 V Underflow
As of a decimal value of 32512, the value acquired is beyond the overrange and is no longer
valid. If this is the case, the error status "Overflow" is diagnosed. The overrange is a
tolerance range before overflow is reached.
As of a decimal value of -32513, the value acquired is below the configured measuring range
and is no longer valid. If this is the case, the error status "Underflow" is diagnosed. The
underrange is the equivalent of the overrange, but for negative values.
An "Overflow" or "Underflow" diagnostics may be caused, for example, by a wire break, an
incorrect measuring range or incorrect wiring.
Note
Accuracy
The accuracy specified in the technical data of the relevant module is only guaranteed within
the rated range.