Leuze MSI 100 Safety Relay

Hi, I have a project that requires the use of a prgrammable safety relay, so I decided to use a Leuze Electronic MSI100 Programmable Safety Relay. These units are very easy to programme using the Safe Function Blocks and the Other Safe Functions (Like And, Or, Not).

But what has got me stumped is how are these any different to a standard PLC that has been programmed in the same way.

I have used other non programmable safety relays, and these relays do have the functions that check themselves for instances of shorting between the input P and N channels due to the integral design, but the safety PLC that I am using cannot do this feature because all the inputs are PNP. Am I missing something here?

Could any of you smart people explain to me the facts of life about Programmable Safety PLC's .

Best Regards

Joe