Rockwell Automation T3418F ICS Regent Monitored Digital Input Modules, Field Powered (Type F) - Manual (Page 15)

Monitored Digital Input Modules, Type F (T3411F, T3418F)
P D - 6 0 2 7 M a r - 0 6
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Figure 9 shows a single switch connected to a redundant
monitored input modules. This type of configuration uses a
line monitor device which contains two zener diodes, one in
series with the field switch and one in parallel with the field
switch.
Figure 9. Line Monitoring, Single Switch Connected to
Redundant Modules.