It is not recommended to connect the two common points because they are separate supplies and the ratings of the two inputs are different.
Connecting the two Commons essentially connects two power supplies together. This is not necessarily a problem until other connections (S1 & D1, etc) are made, in which case there could be over voltage situations depending on what is connected to S1 and D1.

e.g. You decide to externally excite the Digital inputs:

• The onboard Digital Inputs (S inputs) can handle up to 150V but the expansion card Digital Inputs (DI inputs) can only handle 50V. If the Commons are tied together and a 100V supply is connected, the Expansion card Digital Inputs could get damaged.

Original article#16491
Last Revised: November 1, 2007
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All content © 1992-2007 Schneider Electric

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Legacy KB System (APS) Data: RESL189280 V1.0, Originally authored by KBAdPM on 11/17/2007, Last Edited by KBAdPM on 11/17/2007
Related ranges: ION7550/ION7650