Case Studies

Dulles Airport Train Terminal

Redundant SCADA system for Train Tunnel Ventilation

Posted in HMI and SCADA, Infrastructure, Manufacturing Automation and Intelligence, PLC Programming, Siemens PLC


DMC programmed a redundant SCADA (supervisory control and data acquisition) system to control the train tunnel ventilation at a major international airport. The system's main objective is to control, reduce, and help extinguish a fire if it occurs in the train tunnel.



There are certain control systems that cannot be disabled or shut down, even momentarily, because of the threat to people’s lives. In these applications, a redundant control system must be in place to take over instantaneously if the primary system fails.

DMC programmed a redundant SCADA system to control a train tunnel ventilation system. Twenty S7-300 PLC's control about 4000 IO (Input/Output) points. These IO points consist mainly of fans and dampers distributed throughout the train tunnel. The S7-400H Redundant PLC communicates with these twenty S7-300 PLC's via TCP. The S7-400H PLC also retrieves train information data through Modbus TCP. In the event of a fire, the redundant PLC will determine which fans and dampers to actuate based on train information and operator input.

A redundant HMI system was programmed with Siemens WinCC. The HMI displays the process graphically, logs errors, operator interactions, events, and alarms into various databases.

Using IEC 61131-3 programming principles, a simulation mode was easily implemented in the system. This mode simulates the entire system and runs in parallel to the actual physical system. It permits operator training without turning on actual fans and dampers.  If an incident occurs, the simulation mode immediately switches back to the live system and alerts the operators.

Learn more about DMC's HMI and SCADA programming services.

Customer Benefits

  • Redundant fail-safe backup through use of Siemens redundant SCADA system
  • Simulation mode allows operator training on a system identical to the real one without turning on actual fans and dampers.


  • Siemens WinCC
  • Siemens S7-400H Redundant PLC
  • Siemens S7-300
  • Modbus TCP
  • IEC 61131-3