Siemens S7-300 Installation And Operating Manual page 244

Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
Current measurement 0/4 to 20 mA with 2-wire and 4-wire transducer, transducer not redundant, for
application 4
Six process signals can be connected to two redundant modules in this interconnection
variant. One sensor is connected via one channel to the two modules for each process
signal. Sensor supply Vsn is provided by the module for 6 channels. The sensors can also
be connected to an external sensor supply (see Figure
transducer for SM 336; F-AI 6 x 0/4 ... 20 mA HART
diagram and block diagram (Page 224)".
You can use interconnection schemes E to H for this application.
Figure 9-31
For information about the Zener diodes, refer to chapter "Calculation of the residual supply
voltage at the transducer (Page 239)".
To achieve SIL3/Cat.4/PLe using this wiring, you must use a suitably qualified sensor.
Note
You can supply the two F-SMs from two power supply units. If you use two power supply
units, the two grounds on the F-SMs will have to be connected with each other.
244
1oo1 evaluation, redundant F-SMs, single-channel transducer
WARNING
External sensor supply, 2-wire
in chapter "Properties, front view, wiring
Installation and Operating Manual, 01/2010, A5E00085586-10
Fail-safe signal modules