Siemens SIMATIC ET 200AL System Manual page 32

Figure 5-3
Connection of a digital output with a digital input
Pay attention to the potential group
When a digital output is connected to a digital input, pay attention to the potential groups.
Depending on the configuration, 1M and 2M may then be connected, resulting in
elimination of the electrical isolation between 1L+ and 2L+. The possible compensating
currents can destroy the digital module.
Distributed I/O system
System Manual, 08/2019, A5E31861578-AG
Potential conditions of the ET 200AL distributed I/O system (Part 2)
WARNING
5.3 Electrical configuration of the ET 200AL
Connecting
31