the invention relates to a circuit layout for a serial transmission and reception interface , in particular for a vehicle occupant protection system , as well as a procedure for controlling the same . existing central units , in particular vehicle occupant protection systems , featured in most cases separate serial transmission and reception interfaces for the various modules to be connected , causing costs to increase whenever an additional diagnosis module is to be connected . by means of a switchable circuit layout for the interface , this will be connected either to a communication module or to the diagnosis module . the circuit layout is particularly simple and space - saving , being set up by means of diode switches , and preferably features a potential tapping point for automatic switchover into diagnosis mode .