An extender system for MIL-STD-1553 buses transparently couples bus components at multiple developer sites. The bus network extender is a relatively inexpensive system that minimizes the time and cost of integration of avionic systems by providing a convenient mechanism for early testing without the need to transport the usual test equipment and personnel to an integration facility. This bus network extender can thus alleviate overloading of the test facility while enabling the detection of interface problems that can occur during the integration of avionic systems. With this bus extender in place, developers can correct and adjust their own hardware and software before products leave a development site. Currently resident at Johnson Space Center, the bus network extender is used to test the functionality of equipment that, although remotely located, is connected through a MIL-STD-1553 bus. Inasmuch as the standard bus protocol for avionic equipment is that of MIL-STD-1553, companies that supply MIL-STD-1553-compliant equipment to government or industry and that need long-distance communication support might benefit from this network bus extender.

The state of the art does not provide a multicoupler source for this purpose. Instead, the standard used by the military serves merely as an interface between a main computer in some device or aircraft and the subsystems of that device or aircraft — for example, a subsystem that controls wing flaps or ailerons. Unfortunately, the transmission distance of a state-of-the-art MIL-STD-1553 system is limited to 400 ft (122 m). The bus network extender eliminates this distance restriction by enabling the integrated testing of subsystems that are located remotely from each other, without having to physically unite those subsystems. Interlinking by use of the bus network extender is applicable to 90 percent of all required testing for the military; hence, it offers the potential for savings in cost and time. There is also potential for commercial applications in simulation and training and in the development of real-time systems.

The bus network extender enables long-distance communications by use of specified media and compliant equipment, while conforming to all relevant MIL-STD-1553 specifications, including those that pertain to response times and data formats. The bus network extender can also insert simulated or table-driven data addressed to any bus controller (BC) or remote-terminal (RT)/support-area (SA) network combination.

In a system under test, subsystems connected at a local site are connected to the bus network extender. After the connection is made, the output of a remote bus controller mimics the activity of the bus controller at the local site and can be configured to operate with multiple remote sites. The entire bus network is then controlled by a workstation coupled at the local site. The configuration of the bus network extender is controlled by a single text file that is created by the user and that can be modified at the local site.

In order to extend a network, the bus network extender effects a logical sequence of events. The BC (either simulated or hardware in the loop) first sends a MIL-STD-1553 message to the RT. The RT passes the message to a central processing unit (CPU). A master CPU reads the message and transmits an Ethernet (or equivalent) MIL-STD-1553 message to a slave CPU, which receives the message and begins building a MIL-STD-1553 BC message chain. The slave CPU then sends the MIL-STD-1553 message to the RT via BC hardware, a response to the message is received by the BC hardware, and the CPU is notified. After notification, a slave CPU returns the MIL-STD-1553 message response to the master CPU over the Internet. If the message is a "transmit" message, the master CPU fills the destination RT data buffer with response data. Finally, new data become available to the hardware-in-the-loop or simulated BC.

The bus network extender also provides the following:

  • An RT/SA address can be mapped to a different RT/SA on the remote RT to avoid having to change jumpers or reprogram an address.
  • The system can be set to respond to some SAs by simulating other SAs. This capability enables testing even when some sensors or devices on a remote unit under test are unavailable.
  • Multiple MIL-STD-1553 buses can operate simultaneously.
  • Common configuration files can be used to control and document the interfaces to the bus-network-extender interfaces.
  • Responses to MIL-STD-1553 messages can be generated in real time.
  • It is possible to perform byte swapping for computers or firmware controllers that are based on different processor architectures.

Performance depends on the type of communication channel used to connect local and remote sites. In a compromise, commands are buffered by the bus network extender and appear, at least to the BC, to satisfy response-time requirements. However, data supplied by the bus network extender to the BC are transmitted from one or more frame times in the past. The system thus operates correctly with respect to protocol and timing but is subject to a delay in data content ranging from tens of milliseconds to seconds. Such delays result from finite signal-propagation speed and are unavoidable. Notwithstanding such delays, the bus network extender is well suited for protocol and interface integration testing, even on slower links. On faster links, some systems equipped with the extender can cycle MIL-STD-1553 frames at full speed, subject to only inevitable data delays. Some systems equipped with bus network extenders may seem to operate in real time on high-speed links.

The bus network extender is equipped with utility software that, upon command, displays the status of all BCs, remote transmission queues, and RT message queues. The software also provides a configuration file for all connected systems to provide error-free configuration at all locations, a configuration file that contains an entire system interface control document, and an input file that performs extensive error checking. Since slave BCs are configured automatically, remote configuration data are totally eliminated when remote simulations are not required. Should a slave network server be unreachable, the extender attempts to re-establish network connections automatically while maintaining adherence to real-time-response requirements for all MIL-STD-1553 messages.

This work was done by Julius Marcus and T. David Hanson of GeoControl Systems, Inc., for Johnson Space Center. For further information, contact the Johnson Commercial Technology Office at (281) 483-3809. MSC-22741

NASA Tech Briefs Magazine

This article first appeared in the November, 2003 issue of NASA Tech Briefs Magazine.

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