Developing an Auto-Synchronized Multi-Avionics Protocol Data Bus Acquisition System

A modern aircraft includes multiple intelligent subsystems that aid in communication, navigation, and surveillance. These subsystems, called line replaceable units (LRUs), are modular and easy to replace in the field. Typically, independent teams design and develop LRUs. It is important to test the LRUs with a test rig on the ground before integrating them on the aircraft. A ground-based LRU integration rig (LRUIR) facility uses state-of-the-art hardware and software in a distributed architecture to help completely integrate the process of verifying the functionality of the avionics subsystems. An LRUIR provides a platform to test the LRUs in an integrated environment on the ground.

System implementation schematic diagram.
Real-time data acquisition of the test rig is important to ensure reliable testing of the LRUs. The data acquired provides insight into the behavior of the units and the communication among them. A large number of LRUs connect to the data bus acquisition system, which features 730 I/O points from different LRUs, user-configurable data formats for acquisition, different protocols for different data formats, and multiple chassis.

This was a validation system, which made efficiency and calibration the top priorities. National Instruments’ Measurement & Automation Explorer was used to first test the individual cards, applying the concept of self-test to make sure the system checked all the cards as soon as the software ran. The following cards were used in the system:

  • NI PXI-6225 M Series DAQ board for analog acquisition
  • NI PXI-6511 module for digital acquisition
  • NI-8431 device for asynchronous RS-422 measurements
  • NI PXIe-7962R FlexRIO module for synchronous RS-422 measurements
  • NI PXIe-8234 Gigabit Ethernet interface device
  • Avionics Interface Technologies (AIT) MIL-STD-1553 for MIL acquisition
  • AIT ARINC 429 module for ARINC acquisition

Next, the user could design each parameter from low-level to high-level configurations. Low-level configurations included sampling rates for analog or transistor-transistor logic, and open source for digital, start bits, stop bits, RS-422 baud rates, MIL real-time addresses and sub-addresses, or port numbers in case of Ethernet. High-level configurations included encoding techniques such as non-return-to-zero, biphase, or even protocols like highlevel data link control in which the user could configure for start of frame, end of frame, controls bits, or FCS.

The main challenge was to synchronize up to a millisecond among different cards using different protocols spread across three chassis. This was not a single-step process. Any event occurring during the acquisition process had to be captured by all three PXI systems with the same timestamp so the event could be recognized during analysis. First, the network was synchronized with the GPS time. Next, network time protocol (NTP) was used to synchronize other systems within the network. Finally, precision time protocol (PTP) was used to synchronize the three chassis. NI-Sync helped avoid traditional coding techniques for time stamping each event, which would not have been as precise.

Inspired by NI Diadem data management software, the new custom software was made user friendly so the user could view the acquired data as a graph or table, or with special templates for MIL and ARINC data. A customized coding format was developed for synchronous RS-422 using the FlexRIO FPGA module. The FPGA Module offered extra flexibility for customizing the format per the client’s requirements.

The application software was divided into two modules — the real-time module and the host user interface module. The real-time module was deployed in the PXI controller, and the host user interface module ran on the client PC. The TCP/IP Ethernet interface facilitated communication between these modules.

The host system ran the LabVIEW application software, which featured:

  • Login: Only authorized users could access the software.
  • Configuration: Users could configure parameters such as sampling rate, baud rate, or conversion parameters for the desired channels present in the hardware.
  • Acquisition: Users could see the data acquired in real time.
  • Diagnostic: Users could test the functionality of channels.
  • Analysis: Users could convert the data stored during acquisition into .xls format so the user could open the files using Microsoft Excel.

The real-time system used:

  • NI VISA to acquire asynchronous RS- 422 data
  • NI LabVIEW FPGA module to acquire synchronous RS-422 data
  • AIT drivers to help acquire ARINC and MIL-1553 data
  • User Datagram Protocol (UDP) to acquire Ethernet data

The post analysis of data was done using NI Diadem data management software.

LabVIEW was chosen to program this system because of its excellent graphical capabilities and tight integration with NI PXI products using NI-DAQmx measurements services software.

This article was written by Swati Poduval of Captronic Systems Pvt. Limited (Bangalore, India), using products from National Instruments (Austin, TX). For more information, Click Here .