Abstract: A security translator (STL) that serves for connecting a data network (N2) pertaining to a lower security level to a data network (N1) pertaining to a higher security level examines its own integrity at definite intervals in order to ascertain malfunctions and, where appropriate, to trigger a transition to the secure failure state. The secure failure state consists in the connection from the data network with a lower security level to the data network with a higher security level being interrupted. This is obtained by means of a watchdog circuit (WD1, WD2) which keeps the connection (CTN) between the data network (N2) with a lower security level and the data network (N1) with a higher security level closed as long as it receives regular signs of life and which automatically interrupts the connection if it receives no sign of life over a predetermined period.