Hi,

I've got a 1214C DCDCDC and all M are common. I've checked with a multimeter. M to power the CPU, M from souce and M of inputs.

But today I was wiring two 1215C DCDCDC and M aren't common. None.

Is that so? Any reason?

Thanks in advance

According to the2014 (1AE31) and 2022 (1AE40) manual.

All 1200 series have sinking or sourcing inputs. 

in the manual:

(2) For sinking inputs, connect "-" to "M" (shown). For sourcing inputs, connect "+" to "M". 

 
M is common the group of the inputs. 
It does not mean, with this 1200 series PLC that M =  ground.

M of the inputs can be common positive or common negative.