Designing a PXI Instrument for Testing & Simulation of Avionics

Designing a PXI Instrument for Testing & Simulation of Avionics Ethernet Systems Troy Troshynski, Avionics Interface Technologies Overview More and more aircraft systems are turning to the use of Ethernet based networks in order to cope with the demand for higher data volumes while at the same time controlling costs and complexity. Ethernet based networks take advantage of a common and ubiquitous technology that comes with a built-­‐in ecosystem of tools and equipment. Additionally, because this technology is so commonly used also outside of aerospace and avionics, there are a wealth of human resources and expertise available with knowledge of the technology. Avionics applications which utilize Ethernet based networks differ from more traditional Local Area Network (LAN) applications in some key ways. For LAN applications, plug-­‐and-­‐play operation is a key driving feature. Network operators want to be able to plug in new network nodes and have them automatically join to the network and become operational. Avionics networks are different because they are very statically configured and new network nodes are only added during upgrades to the aircraft systems. As a result, Avionics applications of Ethernet networks are typically very streamlined in terms of the protocol stacks used between the network interface and the client applications using the network services. For example, Avionics applications typically don’t utilize services like Dynamic Host Configuration Protocol (DHCP) or many of the services provided by Internet Control Message Protocol (ICMP) because these types of protocols are used over Ethernet to support the plug-­‐and-­‐play and dynamic network configuration services which are not required. Another key difference between an avionics Ethernet network and a LAN is the nature of the data traffic carried in the network. On a typical LAN, communications can be characterized as asynchronous and NOT periodic. That is the network is used for unsynchronized communications like website requests, and file transfers which are triggered by the needs of human users of the network. By contrast, Avionics Ethernet networks largely carry data exchanged between sensors, actuators, and control system computing nodes. This data traffic is by nature very cyclic, periodic, and scheduled. In order to support Avionics Ethernet test and simulation applications, an Ethernet PXI test instrument must have several features to be able to support the unique properties of avionics applications. Periodic Scheduled Data Transmission A typical use of a PXI test instrument in an avionics test application is to use the instrument and the host system (processor) to emulate all of the aircraft system inputs to a Unit Under Test (UUT), for example a flight control system or flight deck display system. In this type of application, the PXI test instrument must be capable of emulating a large amount of periodic data. To support this, it is important the test instrument have the on-­‐board capability to schedule and generate dynamic data to simulate the outputs of multiple sensors. This requires the Ethernet test instrument to provide onboard computing resources, such as one or more embedded processors. Embedded Protocol Stack Because the avionics Ethernet test system must support the simulation of multiple aircraft systems and/or sensors during a simulation, it is desirable to push as much processing out to the Ethernet test instrument as possible. Therefore, an avionics Ethernet test instrument must implement the whole stack of upper layer protocols, which could include IP, UDP, ARP, RTP and others onboard in order to offload from the host system. Because avionics applications typically used profiled (simplified) versions of the upper layer protocols, they can typically be implemented in FPGA firmware onboard the test instrument. Synchronization With Legacy Avionics Data Busses Almost all avionics platforms which utilize Ethernet for data communications also still employ legacy avionics bus technologies like AIRNC 429 and MIL-­‐STD-­‐1553. As a result, any complete test system for avionics networks must also provide interfaces for these legacy bus protocols. Typical test scenarios often require synchronization of the data simulated on these legacy data busses and on the avionics Ethernet network. To support this the Ethernet test instrument must utilize key PXI features such as the PXI 10MHz clock and Star Trigger. Additionally, the Avionics Ethernet test instrument must also support time synchronization technologies, such as IRIG-­‐B, which are common in avionics applications. PXI Express Avionics Ethernet Test Instrument The AIT PXIe-­‐664 test instrument provides dual Ethernet interfaces capable of operating at 10, 100, and 1000 Mbit/sec network speeds. It utilizes the Small Form Factor Pluggable (SFP) Ethernet transceivers in order to easily support either copper or optical Ethernet physical layers. The module also utilizes the PXI 10MHz and Start Trigger signals for synchronization with other instruments within the PXI chassis. The module also provides 2 onboard microprocessors to support the generation of dynamic, periodic data streams and a full onboard UDP, IP, and RTP protocol stack. Figure 1 AIT PXIe-­‐664 Avionics Ethernet Test Instrument