Abstract: A terminal (10) in a data exchange system with exchangeable portable user data carriers (4) has for their connection at least one connecting unit (3) and has further at least one data input unit (5). Exchangeable control modules (8, 9) are connected through one or more other, preferably similar, connecting units (6, 7). The modules are equipped with microprocessors and contain mechanically and cryptographically protected memory areas which receive secret data and keys for the secure carrying out of identity and authenticity tests, as well as application specific operational programs. These construction methods offer high security against unauthorized data access as well as against fraudulent manipulation of the device. They also greatly simplify the application independent design and construction of the terminal.