Siemens S7-300 Installation And Operating Manual page 187

Sensor supply
Voltage dips of the power supply are not buffered by the module and, thus, affect the
sensor supply.
This can cause the measured value to be false.
You can avoid voltage dips by using a voltage supply according to the NAMUR
recommendation (see Chapter " Safe Functional Extra-Low Voltage for Fail-Safe Signal
Modules (Page 44) "). Alternatively, use a transducer with an appropriate battery backup or
diagnostics.
External sensor supply
The figures below shows how you can supply power to the sensors by means of an external
sensor supply (for example, from another module: 1L+).
Figure 9-5
Figure 9-6
Fail-safe signal modules
Installation and Operating Manual, 01/2010, A5E00085586-10
WARNING
External encoder supply, 2-wire transducer for SM 336; AI 6 x 13Bit
External encoder supply, 4-wire transducer for SM 336; AI 6 x 13Bit
Analog modules
9.2 SM 336; AI 6 x 13 Bit
187