This SAE Recommended Practice establishes the requirements for a Class B Data Communication Network Interface applicable to all on and off-road land based vehicles. This document defines a minimum set of data communication requirements such that the resulting network is cost effective for simple applications and flexible enough to use in complex applications. Although this document addresses all seven layers of the OSI model, it primarily focuses on the Network, Data Link and Physical Layers. Taken in total, the requirements contained in this document specify a data communications network philosophy that satisfies the needs of automotive manufacturers.
Although the higher layer OSI requirements are essentially identical for all networks defined by this document (see Section 3.3, Figure 1), differing data rate requirements necessitate the use of different physical layers. Therefore, one physical layer is optimized for a data rate of 10.4 Kbps while the other physical layer is optimized for a data rate of 41.6 Kbps. Additionally, this document outlines some physical layer alternatives to the two defined implementations that may allow the network to operate with different characteristics (e.g. data rate).
Although devices may be constructed that can be configured to operate in either of the two primary implementations defined herein, it is expected that most manufacturers will focus specifically on either the 10.4 Kbps implementation or the 41.6 Kbps implementation, depending on their specific application and corporate philosophy toward data link usage. However, low volume users of network interface devices may still find it more effective to use a generic interface capable of handling either of the primary implementations specified in this document.
Designers of integrated circuit(s) associated with these requirements are encouraged to innovate and add flexibility in their designs in order to gain broader market acceptance of their specific product offerings.
While two implementations are fully characterized here (i.e. 10.4 Kbps and 41.6 Kbps), two other implementations are being pursued and will be proposed in a future version of this document. These implementations may be generally characterized as:
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a
125 Kbps, with possibly different bit encoding, drive type, media, and redundancy characteristics than the current two implementations; and,
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b
41.6 Kbps PWM utilizing balanced current drive, no redundancy, two wires and employing bus bias voltage as a wake-up signal.
This SAE document is under the control and maintenance of The Vehicle Network for Multiplexing and Data Communications (Multiplex) Committee. This committee will periodically review and update this document as needs dictate. In the document’s present form, as a recommended practice, it contains information that is intended as a guide to standard engineering practice. It is the eventual goal of this committee to upgrade this document to an SAE Standard. To accomplish this upgrade, the document must contain information describing a broadly accepted engineering practice. This means hardware must be developed requiring most likely refinements, technical advancements, and a broad acceptance of a single cohesive set of network features. As this recommended practice matures into a fully accepted standard, it is recognized that conformance verification procedures will need to be established.