Siemens SINAMICS S120 Manual page 446

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8.22 K95, Control Unit CU320-2 PN
Note
Power supply for the digital inputs
In the circuit example, the power for the digital inputs (terminals -X122 and -X132) is
supplied from the internal 24 V supply of the Control Unit (terminal -X124).
The two groups of digital inputs (optocoupler inputs) each have a common reference
potential (reference ground M1 or M2). To close the circuit when the internal 24 V supply is
used, the reference grounds M1 / M2 are connected to internal ground (M).
If power is not supplied from the internal 24 V supply (terminal -X124), the jumper between
grounds M1 and M or M2 and M must be removed in order to avoid potential current loops.
The external ground must then be connected to terminals M1 and M2.
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Cabinet Modules
Manual, 03/2013, A5E03263538A