The message passing scheme shared by all XDS boards is simple, and designed to conserve both scarce memory and I/O addresses, and the even more limited interrupt capabilities of the PC. Up to sixteen XDS boards of any type can share a single pair of I/O locations, a 32K block of memory, and a common interrupt line. This is an important consideration when trying to design a system with a variety of boards from different manufacturers. The XDS boards are designed to conserve resources, allowing the inclusion of other third party boards in the system.

The message structure is designed to be both understandable and easy to use. Port control is accomplished through a set of simple commands and responses. Commands perform higher level functions such as connect, hold, and disconnect rather than requiring separate instructions for seizing ports and creating audio paths. This minimizes the application overhead. Similarly, responses from the board indicate state changes, rather than the values of sensors. Thus the board will report that ringing has been detected or that a port has gone off-hook. The firmware on the board takes care of any qualification of signals needed, of any debouncing of sensors, or timing. This isolates applications from the details of line behavior. And because the board reports state changes asynchronously, the reaction of the application is not as time critical as if it had to poll sensors on the basis of a timed interrupt.

In addition, some XDS boards allow connections between different buses. For example, the XDS Switch Matrix/Conference board can serve as a switching element connecting boards equipped with MVIP, PEB, and APIB (Amtelco PCM Interface Bus) buses. This allows the developer to pick the mix of boards that best suits the requirements.    

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