Communications Monitoring and Emulation with MIL-STD-1553B and RS-422

Summary

Communications emulation and monitoring is a core part of all avionics test projects in aerospace, but frequently involves painful editing, difficult troubleshoot, and complex requirements that aren’t easily met by a fully automated solution. As part of the SLS NASA BOLE project, DMC designed and developed a solution for HIL testing MIL-STD-1553B and RS-422 communications of the Booster avionics that not only provided an interface for simply updating message tables and scaling parameters, but also provided appropriate logging and simplified hardware architecture for maximum troubleshooting capability with minimal manual parsing.

DMC developed an integrated Real-Time and PC software stack that provided lossless bus traffic logs, emulated the redundant synchronized bus controllers of the SLS Flight Computer, and provided a means to programmatically send commands to test Booster avionics handling of arm/disarm commands, flight profiles, and faults. DMC also developed custom signal routing interfaces to accommodate each test configuration and distribute an IRIG time synchronization signal throughout.

Customer Benefits

• Automated Configuration – gives the user the ability to verify harnessing and communication settings, as well as configure message tables and scaling, all within a clear and unified software interface
• Avionics Verification – synchronized commands and monitored signals allow verification of avionics response times, functionality, and fault handling
• Synchronized Control – pre-programmed actuator profiles and emulated commands are sent from automated real-time sequences
• Maintenance and Troubleshooting – manual command interfaces for jogging actuator positions and sending LRU commands to assist with maintenance operations and ground testing
• Logging and traceability – high fidelity bus logs containing full traffic data and synchronized timestamps for test analysis and diagnostics allow the user to see all of their comms data in a clear, lossless way

Technologies (Hardware & Software)

• AIM MIL-STD-1553B PXIe Modules
• Astronics Ballard Omnibus II RS-422 PXIe Modules
• NI PXI Chassis and controller
• NI LabVIEW
• NI VeriStand

Solution

Technical Challenges

The Booster communications specification presented the DMC team with a uniquely large and technically complex set of challenges:

• Refining and translating a large requirement set into integrated hardware and software solutions
• Understanding and supporting all testing needs of different client teams at all stages of Booster development, qualification, and assembly
• Emulating the SLS Main Stage Flight Computer with microsecond-level synchronized control of 3 MIL-STD-1553B Bus Controllers
• Support monitoring and emulation of RS-422 buses in various hardware configurations
• Integration into an application providing programmatic and manual control, real-time data viewing, and alarms

The DMC team was able to overcome these challenges with weekly technical meetings with traceable recorded discussions with the client to maintain understanding, and a formal phased project approach of design phases, development, and testing. DMC and the client were able to work in tandem to make sure the solution we built gave them all the capability they needed in as simplified a way as possible; DMC’s core value of Share Information really guided this aspect of development to make sure everyone had what they needed.

Core Technical System Design

RS-422 Monitoring

RS-422 protocol is used in some of the booster avionics. The test station needed to monitor this traffic without disrupting the signal flow.

DMC’s solution gives our client a way to flexibly monitor at different points within their system and between different LRUs without having to remate cables, saving time when diagnosing issues and comparing internal bus traffic.

For this traffic snooping, an RS-422 cable harness is attached to the interface connector designated for monitoring. The 422 signals are routed into small stubs so as not to introduce reflection on the signal line, and then returned back out to the booster system. Two Rx pairs on the RS-422 transceiver card are used to monitor both sides of the signal. The Ballard Omnibus II card has an IRIG B input for time synchronization with the rest of the system.

In the automated breakout box, there are two DPDT relays in series. This relay configuration allows the signals to be routed to the test station and back out for emulation and monitoring, or directly passthrough. When the two relays are misaligned (one set to pass-through and one set to the test station), then an open circuit fault is induced.

DMC developed a VeriStand Custom Device to handle the traffic reading, logging, and decommutation. There are multiple distinct RS-422 buses in the booster, and the custom device is built to handle these different configurations and changes so the customer can handle all of their buses with one unified software.

RS-422 Emulation

In some test cases, the test station needs to be able to emulate the RS-422 communication of portions of the avionics to diagnose subsystems of the booster. To emulate, a cable harness is connected to the test station interface on a dedicated emulation interface connector.

As mentioned above, the automated breakout box is used to route the signals into the configuration on the right for emulation.
The same RS-422 VeriStand Custom Device that handles monitoring also handles emulation. The messages to be sent are configurable and accommodate several baud rates.

This emulation mode allows the client to diagnose specific issues within subsystems of the booster by using our system to emulate not just the flight computer, but also internal avionics LRUs.

MIL-STD-1553B Command & Monitor

The test station is capable of monitoring and emulating MIL-STD-1553B bus traffic as a bus controller. DMC’s test station mimics the behavior of the flight computer talking to the 1553 Devices within the avionics of the booster. The automated breakout box pictured adds fault insertion to either side of the signal for testing as needed. The architecture is expandable to multiple redundant buses.

On the software side, DMC developed a VeriStand Custom Device to appropriately receive and command the messages in the bus schedule, as well as decommutate them into meaningful parameters. The VeriStand Primary Control Loop runs at the minor frame rate to ensure we met the speed requirements for the buses. The AIM 1553 Transceiver card also has an input for IRIG B, which allows synchronized timestamps on all the data.

The bus schedule, message tables, and decommutation conversion equations are configurable, so the client can easily make changes during the booster development.

Conclusion

Testing is critically important in aerospace applications as minor issues can lead to rework, time delays, and/or catastrophic consequences. Integration of communications emulation and monitoring into a highly-featured application provides the means to diagnose issues and test avionics at various stages of Booster assembly and integration, all within a single application. Performing consistent tests and reviewing comparable, high-fidelity bus traffic logs gives confidence that functionality isn’t compromised at any step of the process.

Learn more about DMC’s Test & Measurement Aerospace and Defense expertise and contact us for your next project.