Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.14 Diagnostics
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
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No supply voltage L+
You can diagnose a missing or low supply voltage L+ at the analog module by activating the
"No supply voltage L+" checkbox. If the supply voltage is missing or too low, the status and
error display on the analog module will provide the relevant information. The entry in the
diagnostics buffer also makes this information available to the CPU.
If there is no supply voltage, no other types of diagnostics available are possible either.
Wire break
The term wire break denotes a failure state that is triggered by the interruption of a usually
closed electrical circuit.
Wire break in analog input modules
The "Wire break" diagnostics function facilitates the detection of errors during system
operation. In STEP 7 you define the module properties by setting various parameters.
Depending on the module used, you may, for example, be able to set a current limit
parameter at which a wire break is to be reported.
Figure 2-19 Configuring the wire break current limit for an analog input module in STEP 7
Usually, the live zero technique is deployed to detect wire breaks. The start of the measuring
range for the standard live zero signal of 4 to 20 mA is assigned the 4 mA signal value. A
wire break between the input and sensor can therefore be detected based on the missing
current flow. Instead of a current signal, you may also use a voltage signal from 1 to 5 V to
detect a wire break. The "Wire break" diagnostics function can therefore only be configured
for a range of 4 to 20 mA or 1 to 5 V.
For "Resistor", "Thermal resistor" and "Thermocouple" measurements, a current is
impressed into the line. If a wire break occurs, the flow of current is interrupted and the
analog module therefore detects the wire break.