A method for safe data communication via WAN, LAN (e.g. Internet) between at least one external access unit and a field device, or a field bus adapter for determining or monitoring at least one physical or chemical process parameter. According to the method unauthorized accessing of a field device, or a field bus adapter, in the field is blocked in that the operator of the field device or the field bus adapter permits the external access unit to have targeted access to the field device, or the field bus adapter.