Abstract: A reading device having a reading head for automatically reading an external non-stationary data storage arrangement, for example, a credit card, may be supervised continuously with respect to its readliness for service. When no external data storage arrangement is inserted, a stationary test data storage arrangement is read by the reading head. The stationary test data storage arrangement is the same or similar to the non-stationary data storage arrangement and can be read identically by the reading head. If the test data read by the reading head from the test data storage arrangement are not transmitted to a processing unit at an expected time, a defect signal is set.