ARINC 661 was created to provide a standard set of definitions for a cockpit display system (CDS). It separates the display mechanism from user applications in managing aircraft avionics functions. This human machine interface (HMI) allows a new user interface to be implemented without modifying the user application. XML-based interface definition files (DFs) allow changes to be easily incorporated. ARINC 661 is often utilized in applications requiring DO-178B certification. The XML DF is normally converted to a binary representation.

The ARINC 661 standard is almost 10 years old and has been updated on a regular basis. Its first use was on the Airbus A380. The interface uses a client/server approach similar to X-Windows with a client application (CA) communicating with a display server or CDS. This split makes it easier to distribute user interface and user application work. It is also easier to place the display away from the application location.

The standard is supported by human-interface and graphical-user-interface (GUI) tools from a number of vendors, including Quantum 3D (www.quantum3D.com) and Presagis (www.presagis.com). There is even an open-source ARINC 661 display server written in Java and supported by the FlightGear open-source flight simulator (version 2.0 is currently available for download).

The expansion of the number of widgets has been the main enhancement to ARINC 661 over the years. The initial specification had 42 widgets; the latest incarnation, Revision 3, increases this to 68. Simple widgets include shapes like GpLine and GpRectangle. Of course, there are the usual suspects like buttons and list boxes. As might be expected of an aviation interface, ARINC 661 also has more advanced widgets, like those for displaying map information.

Like most window management systems, ARINC 661 can group widgets together in a layered presentation. ARINC 661 also supports multiple windows, but these must be displayed in a non-overlapping layout. ARINC 661 does not specify what widgets look like or how they are implemented--it only specifies how they are defined and what the interaction with the widget is. This allows more flexibility in implementation.

The client/server approach makes remote displays significantly easier. The client and server can reside on the same system running as different threads. A server can typically handle multiple clients, as well, so merging data from different sources into a single display is possible. Likewise, replication of information to multiple display servers is a simple chore. ARINC 661 handles inputs as well as display chores. This can be integrated locally so a cursor movement--with respect to a widget--can cause a display change, such as highlighting a button. Actions that are fed back to the application come in the form of events. The application can affect display changes by sending back messages to the CDS.

ARINC 661-compliant systems are interchangeable. They all operate off of the same XML definitions. The resulting displays and user interaction will be functionally equivalent although their presentation and input devices may differ. This makes it significantly easier to address certification issues when changing or updating hardware and software. Alternative graphical interfaces may not provide this level of support. Most vendors that support ARINC 661 provide WYSIWIG HMI development tools. Many target other platforms, as well, allowing migration of interfaces when necessary.