Siemens SINAMICS S120 Manual page 165

Note
Ensuring the function of digital inputs
For the digital inputs DIx+ to function, the reference potential must be connected to input DIx-
in each case.
This is achieved by using one of the following measures:
1. Providing the ground reference of the digital inputs
2. A jumper between DIx- and terminal M1
Control Units and additional system components
Equipment Manual, 06/2020, 6SL3097-5AH00-0BP2
Terminal Modules
6.6 Terminal Module TM54F
163