This SAE Aerospace Standard (AS) establishes the requirements for the use of IEEE-1394b as a data bus network in military and aerospace vehicles. It defines the concept of operations and information flow on the network. As discussed in 1.4, this specification contains extensions/restrictions to “off-the-shelf” IEEE-1394 standards, and assumes that the reader already has a working knowledge of IEEE-1394.

This document is referred to as the “base” specification, containing the generic requirements that specify data bus characteristics, data formats and node operation. It is important to note that this specification is not stand-alone - several requirements provide only example implementations and delegate the actual implementation to be specified by the network architect/integrator for a particular vehicle application. This information is typically contained in a “network profile” slash sheet that is subservient to this base specification. In a similar manner, the electrical characteristics of the bus media, as well as connector information is contained in a “physical layer” slash sheet, that also may be unique to a particular vehicle application.

In summary, full understanding of this specification requires knowledge of IEEE-1394b standards and access to the physical layer slash sheets and the slash sheets and handbook for the target application.

Purpose

The purpose of this document is to standardize an approach to using IEEE-1394 in safety-critical/mission-critical applications for military and aerospace vehicles. The information herein will be used to assist the design, fabrication, and maintenance of the nodes that interface via the vehicle’s network.

This document is controlled and maintained by the SAE with technical support from Remote Node vendors.

Application

The IEEE-1394 Network defined in this document provides a deterministic, rate based communication protocol overlaid on the existing IEEE-1394 standard capabilities.

Interpretation

The following interpretations shall be placed upon these words, unless stated otherwise, where they are used in this document.

May: An allowed action.

Shall: A mandatory requirement.

Should: A recommended action.

Will: A declaration of intent.

Additions to the IEEE-1394, IEEE-1394a, and IEEE-1394b Standards

Additions include the use of asynchronous stream packets, a fixed frame rate synchronized with a Start Of Frame packet, addition of a Vertical Parity Check, static assignment of channel numbers, pre-assignment of bandwidth, and use of Anonymous Subscriber Messaging.

Use of Asynchronous Streams

Asynchronous streams are used for most communication on the network. Asynchronous and isochronous packets are not required but may be utilized. Architectures and protocols discussed in the context of this document are based on asynchronous streams, unless it is otherwise mentioned. An example of using asynchronous packets would be to allow test equipment to read data from a Configuration ROM. An example of using isochronous packets would be for streaming video and/or audio.

A Fixed Frame Rate

If isochronous packets are not utilized, there is no requirement for Cycle Start packets to be sent at the normal 125- microsecond rate. Instead, this implementation provides a fixed frame rate for synchronization of the network. If isochronous packets are utilized, it is necessary to utilize the Cycle Master function so the 125-microsecond periodic cycle starts are available to isochronous sources that expect them. Figure 18 in 3.3.3.3 shows an example of interleaving isochronous packets and asynchronous stream packets.

Synchronization Via Start of Frame Packets

A Start Of Frame (STOF) packet is transmitted by the Control Computer on each bus at a periodic (e.g. 100 Hertz) frame rate. This packet informs all nodes on the bus that a new frame has started.

Static Assignment of Channel Numbers

Because asynchronous stream packets are, in reality, isochronous packets, the destination of the packet is identified by a channel number. Unlike standard 1394, the channel numbers for destination nodes are not assigned by an Isochronous Resource Manager (IRM). Instead, the channel numbers for each node on the bus are pre-assigned, are application specific, and will be defined as required by the architecture.

Pre-Assignment of Bandwidth

The lack of an IRM requires that bandwidth be pre-assigned, also. Transmit and receive times for each node on the bus are assigned as offsets, in increments of 1 microsecond, from the start of each frame (STOF packet). The offsets for each node on the bus are application specific and will be defined as required by the architecture.

Vertical Parity Check

Vertical Parity Checking (VPC) is performed on the data area of each packet as an adjunct to the Cyclic Redundancy Check (CRC) performed by the 1394 physical layer devices. The VPC provides additional data integrity as the messages progress through the physical and software layers.

Anonymous Subscriber Messaging

Anonymous Subscriber Messaging (ASM) is a protocol, in which, a Remote Node on the network can subscribe to each message that it requires. The ASM software in the Remote Node will forward only the messages to which the Remote Node has subscribed.

ASM is an upper level protocol tailored for the demands of highly modular embedded real time systems operating under a “data push” paradigm. ASM is designed to be independent of lower level protocols and, as such, does not utilize the 1394 header to transport ASM-peculiar information. ASM is tailored to support deterministic, secure, low-latency communication between processors, sensors, instrumentation, and displays in mission-critical applications. It uses Message IDs to decouple the network traffic from physical addresses so application software can communicate without knowledge of network topology.