Mike Echo,
Many thanks for your reply.
You have answered all my questions here..and the one about the ECU has been answered elsewhere.
The query re the door referred to the main cabin entrance door by the way, with my specific interest being as to the form of locking mechanism used. All is now clear however.
The questions were simplistic purely because I needed an overview of the type rather than any specific engineering details ( as such ).
Once again, thanks for your help.