Disclosed is an HMI system comprising at least one mobile operating and monitoring device for the automation components of a technical installation and a radio link for contactless data transmission between the mobile operating and monitoring device and the automation components. A first firewall is provided for securing data transmission from the automation components to the mobile operating and monitoring device while a second firewall is supplied for securing data transmission from the mobile operating and monitoring device to the automation components. The advantage of the invention consists of the fact that bi-directional data traffic on the radio link between a mobile operating and monitoring device and the other automation components of a technical installation can also be secured by using two preferably equally effective firewalls.