The invention relates to an arrangement and method for preventing unauthorised access to an information system via a network. A workstation according to the invention comprises at least two separate processor units (PA, PB) in one and the same housing and a switch unit (16). Part of the processor units (PA) are connected to public data networks. The other processor units (PB), which are not connected to public data networks, are used as work machines proper. The processor units in the workstation have got separate power supplies (21, 22), separate fixed disk drives (9, 10) and hardware interfaces for electrically isolating data transfer between the processor units. The processor units share a display (20), keyboard (18) and a mouse (19) with their associated interfaces. By means of a control switch (17) the user can select the processor unit to work with. In conjunction with the switch unit there is an indicator (23, 24) which indicates to the user the processor unit to which said devices are connected at any given time. An advantage of the invention is that the workstation, which can be connected to public data networks when needed, is secure against unauthorised access so one can safety work and save information on it. Information received from the network is transferred to the protected processor unit in an encrypted form on a diskette or printing paper, for example.
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