Linear Token Passing Multiplex Data Bus User's Handbook
|Publication Date:||1 May 2012|
This document is intended to explain, in detail, the rationale behind the features and functions of the AS4074, Linear, Token-passing, Bus (LTPB). The discussions also address the considerations which a system designer should take into account when designing a system using this bus. Other information can be found in these related documents:
AIR4271 - Handbook of System Data Communication
AS4290 - Validation Test Plan for AS4074
The AS4074 LTPB is the results of wok by the Linear Implementation Task Group (LIT) of the Society of Automotive Engineers (SAE) Avionics Systems Division (ASD).
The standard was developed to address the peculiar requirements of advanced systems projected for fielding during the late 1980s and beyond. These applications may be in the areas of avionics systems as well as ground based vehicles (vetronics) and stationary systems. Among the requirements for which this bus was developed as a solution are:
- Low Data Latency
- High Fault Tolerant
- Distributed Control (data driven systems)
- Quick, independent reconfiguration in the event of a failure
- High throughputs in excess of 50 Mbps
In order to achieve these goals, the task group investigated existing standards, pinpointed potential problems with these protocols in the areas of real-time systems requirements, and proceeded to address the problem areas with protocol solutions. In some cases, these characteristics of the AS4074 standard seem innocuous. In others, they stand out as areas which require an understanding of the performance characteristics in order to properly utilize the standard to the full capability in a system.
This handbook is intended to introduce the reader to the AS4074 standard and explain, in detail, the operation of all facets of the protocol. It seeks to explain the rationale for the specified operation of the state machine which controls the protocol and offers the user an understanding of how to apply this data bus as a solution to a systems data communications problem.
The AS4074 LTPB standard is organized with a section to provide the user with an understanding of the operation of the protocol followed by the detailed requirements which result from that operation. Following a brief discussion of the overall protocol, the Media Interface Unit (MIU) is discussed for both fiber optic and wire (coax) implementations. Next, the actual frame formats and data field definitions are discussed, followed by a discussion of station management functions. Detailed protocol operational characteristics are found in Section 5 of the document, which includes a high level state transition diagram and state transition descriptions.
This document discusses these topics in much the same order as they are presented in the standard. In addition, there is a hypothetical system design section in which the various aspects of data bus design are covered. Some of the topics addressed are: token rotation and token holding timer settings, station address assignment, and bus test considerations for design activities.
Appendix B allows the user to quickly reference between the AS4074 standard and the handbook and, conversely, between the handbook and the standard.
History of the SAE AS4074 Standard:
As early as 1979, members of the SAE who were involved in advanced aerospace systems development, recognized the need for high speed data transfer capabilities. The advent of MIL-STD-1553 had given the designer a flexible tool for overall system integration; however, it lacked the speed and flexibility which is necessary for future systems.
SAE A-2K, the predecessor subcommittee for AS-2, was asked by representative of the Department of Defense, to develop a standard which would address data communications requirements for the time frame of 1990 and beyond. Initial calls to industry and government for inputs to the committee led to the formation of two task groups. These two groups, the High Speed Data Bus Applications and Requirements Task Group (HART) and the Topology and Protocol Task Group (TAP) were responsible for developing the requirements for a data bus to integrate these high performance systems and select an appropriate protocol for development.
As a result of these activities, the subcommittee elected to pursue two related standards. The first, and the subject of this handbook, is the AS4074 LTPB. The other, discussed in a companion handbook, is the AS4075 high-speed ring bus (HSRB). Each of these standards offers specific benefits in areas such as throughput, fault tolerance, and fault isolation. The designer is encouraged to review the standard in light of his particular requirements and utilize the handbooks as a guide in the selection and utilization of these standards.