1. Scope--The Token Slot Data Link is intended to provide periodic, broadcast communications (communication that must occur on a regular, predetermined basis) within a vehicle system.

The Token Slot protocol achieves this by implementing a masterless, deterministic, non-contention Token Slot sequence which is designed to offer a transmit token to all devices (or nodes) without requiring that they respond. After acquiring the token, messages may be sent and verified using a variety of built-in techniques. The token passing slot sequence is then reinitiated by the current token holder.

1.1 Purpose--This SAE Information Report describes the Token Slot Data Link Communication Protocol. It is intended to cover the attributes of the network structure, network management, bus access procedures, message framing, bit encoding, and message delimiting required to communicate on the Token Slot Data Link. These aspects relate to those items that must be standardized excluding the physical implementation. The physical details of the network such as media, line driver/receivers, and semiconductor implementation are discussed generally but are not specified by this document.

1.2 Background--In recent years it has become apparent that future electronic applications within vehicles will need to communicate critical control information among distributed controllers. This control information to be transmitted is typically periodic in nature and must arrive on a regular, predetermined basis. In many cases, it is changing at rapid rates and only sampled information is provided at appropriate intervals (rather than updating the information only when it changes). This must be done at speeds that are sufficiently fast to meet the requirements of the interactive control applications. The Token Slot Network Data Link is designed to provide this capability.

1.3 Design Objectives--The Token Slot Network was defined with the following goals in mind:

a. It must meet the needs for on-vehicle, rapid, periodic communication between computer assemblies.

b. Target applications for this link will be higher speed control applications (typically above 1 Mbit/s).

c. Message maximum latency time delays must be minimal and bounded to assure stability in closed loop control applications. See Appendix A for methods of calculating and predicting message latencies.

d. There must be an open data rate growth path to accommodate future media technologies and demands for increased data traffic.

e. The architecture must be an open system to allow for the addition and deletion of nodes (devices) both in new designs and during dynamic operation. Note that open architecture does not mean that devices can be added to the data link without regard to their affect on message latency times.

It is intended that this link will be used primarily in vehicle applications requiring feedback and control data to be shared among various distributed devices. For example, the link could be used between distributed engine, transmission, anti-lock braking, and power steering controllers. Flexibility has been built into the protocol to also include event driven and block memory transfer data types. This allows vehicles to be equipped with a single data link to cover all communications to avoid gateways and reduce costs, when system and reliability conditions permit.