Hi,

When we using safety DO module in a SIL3 safety function, there would be two kinds of switching: PP and PM.

I'm quite confused where there are two kinds? what's the benefit of these two? Can they achieve SIL3 with similar wiring?

and, if we choose PM, connect to a relay or a drive, does each M shall have a special wiring? or all the Ms can be mixed and then connect to the same GND terminal?

Thanks so much :)

Hello,

In both of them you can conmute from one switch to another in case one of them fails. The main difference is that in PP both output switches are connected in series: one goes connected to the positive terminal and the other to a GND. But, in PM both must be connected to the terminal of the output module. 

By the way, check this FAQ that might be helpful while wiring:

How can safety related shutdown of standard modules be achieved?

Good luck!