Hello,

SMM 6SL3121-1TE21-8AA3 is cancelled in Sept 2014 & its successor poduct is 6SL3121-1TE21-8AA4 & not 6SL3121-1TE21-8AA3 version C.

Now why system does not recognize 6SL3121-1TE21-8AA3? I guess it could be because of Sinamics CU version.

To confirm this, please check 840D SL integrated Sinamics CU version. You can check your NCU MLFB number, its SW version & CU details in PLC hardware project. 

HTH.

PauloNDS

The NCU version is 720.2.

You need to check this NCU (Sinamics CU) FW version installed. You can check it in PLC hardware project. So with Simatic Manager Step 7 V5.5 or greater go on line & check it.

PauloNDS

The power units.

Should the machine have recognized that new module automatically?

System will detect or recognize the power module in case if it is same, i.e. its version compatibility should match. Before replacing module, user should confirm the module successor product in case if the part is discontinued or cancel. To check it, always contact your nearest Siemens service support center.

HTH.