1. A method for monitoring the data processing and transmission in a data processing unit ( 1 ) having two or more software modules ( 2 A, 2 B) between which data (D) is interchanged, with the data (D) on which the data transmission is based having an associated checksum (P) which, as the data (D) passes correctly through one of selected software modules ( 2 B), is shifted in said software modules ( 2 B) by a number of digits (n) which is associated with this software module ( 2 B).