Mil1394 (AS5643) - A High-speed Connection for Avionics Networks

Mil1394 is an approved SAE standard and is backed by some of the very large subcontractors around the world. Ongoing work within the SAE AS-1A workgroup ensures the continued evolution of the existing Standard.

The technology provides several essential high-lights which have contributed massively to reach its strong market position:

• deterministic behavior
• fault tolerance
• defined latency
• low non-recurring and re-occurring costs
• technology refresh path
• wide industry acceptance

A basic analysis of the AS5643/1394 Beta standards can help designers make their decision about its viability. Each AS5643 bus typically consists of two types of nodes; a Control Computer (CC), also referred to as a Vehicle Management Computer (VMC) and Remote Nodes (RN), also referred to as a Line Replaceable Unit (LRU). Typically, there is only one Control Computer per 1394 bus and multiple Remote Nodes (Sensor, actuators, FADECs, Remote I/O, etc…). Control Computer and Remote Nodes may be connected to optimize the system implementation requirements for weight, volume and redundancy. Weight is minimal, as is volume. 1394 Beta only requires two differential pairs per port, or one TX and one RX per fiber optic port. 1394 Beta allows the system architect to optimize for both weight and redundancy by supporting a mixture of daisy chaining, tree, loop and star topology configurations. 1394’s flexible cable topology support allows the system implementer to optimize cable routing to meet weight/volume and robustness requirements and, unlike star or switched technologies homerun cabling is not required. Designed to be low cost and low power, 1394 Beta may be implemented using industry standard 8b10b SERDES with LVDS-type differential signaling. Both are industry standard for serial I/O and therefore power/performance optimized SERDES implementations are available in both standard silicon and many FPGA families. Currently, almost all aerospace and defense applications use standard COTS 1394 silicon with all the cost and ecosystem benefits that implies; development and production test tools/equipment already exist, multiple software stacks operating under multiple OSs are widely available and, if desired, FPGA PHY and Link IP cores are also available. Additionally, the digital logic in most S400 PHY and Link designs operate at less than 50MHz, which reduce switching currents and help make 1394 Beta FPGA friendly.

AS5643 defines the use of both passive and active transformers to be used with standard 1394 Beta. These transformers provide the electrical isolation necessary to meet the stringent aerospace and defense requirements such as RTCA/DO-160 lightening susceptibility. The active transformers boost the guaranteed minimum differential signal amplitude from 600mV(P-P) to 1100mV(P-P) allowing cable lengths of greater than 30 meters at 500Mb/s to be achieved.

In terms of software requirements, AS5643 architecture keeps Remote Node (LRU) 1394 related software to a minimum. AS5643 architecture specifies pre-assigned static addressing, using 1394 channel numbers, which significantly reduces the plug-n-play requirements of standard 1394 and eliminates the need for device discovery software after power-up and bus resets. The reduced complexity increases robustness by making the software simpler and it allows for communication to resume immediately after a short (1.3usec) or long (166.7usec) bus reset. Simpler software means less lines of code, which means less time and cost to certify the LRU for flight. Finally, the specification is architected to scale with speed and topology, keeping changes to software at a minimum.

Several A&D programs have selected Mil1394 and have incorporated up to 60 devices per airplane. The unprecedented use of a high speed (491.52Mb/s) serial interface has very effectively proven the capability of AS5643/1394 Beta in modern aircrafts, and has opened the door for higher bandwidth communication between the Control Computer and LRUs. While I/O bandwidth is important, system level deterministic behavior is required for most vehicle system networks and AS5643 coupled with 1394 Beta provides the required deterministic behavior.

Using 1394 Beta’s asynchronous stream capability, AS5643 creates a programmable rate-based (time-sliced) protocol that meets the rigorous requirements of advanced aerospace and defense system design all while using COTS silicon. When looked at from a system view, AS5643/1394 Beta is an ideal solution for safety-critical, deterministic (hard real-time) distributed control. As such, it provides the guaranteed latency and jitter needed to ensure that the data required for distributed control functions are delivered in a timely and predictable manner. Today there exists an excellent set of infrastructure products including development tools, test tools and equipment, software stacks, IP Cores and simulation environments. The result is reduced risk, reduced cost, and a reliable, deterministic system with a high level of robustness.