Siemens SIMATIC PCS 7 Configuration page 31

2 Configuring and wiring
2.3 Redundant connections of the distributed I/O
Example of an SM 331; AI 8x0/4...20 mA HART (4-wire transducer)
A 4-wire transducer is connected to two redundant HART analog input modules:
Figure 2–15
The interconnected Zener diodes are required if the system is to continue
functioning while dragging a module.
Configuration of Redundant I/O Modules
Entry-ID: 28430682, V2.0, 03/2015
31