TokenBus

TokenBus is a historical local area network technology that uses a token-passing access method on a physical bus. It was standardized as IEEE 802.4 and designed to provide deterministic communication for distributed control and automation applications. In a TokenBus network, devices are connected to a common bus medium (such as coaxial cable) but operate as a logical ring: a token circulates among stations in a defined sequence, and a node may transmit only while it holds the token. The token can carry priority information, allowing higher-priority devices to gain access sooner and helping to achieve predictable response times in real-time applications.

When a station has the token, it may send a data frame onto the bus, after which the token is released and passed to the next station in the logical ring. The protocol includes mechanisms for error detection, token restoration, and fault handling to recover from contention loss or misbehaving nodes. TokenBus aimed to combine the determinism of token-passing with the simplicity of a shared bus physical layer.

Applications and adoption: TokenBus found use in industrial automation and process control environments where predictable timing was important. However, it faced strong competition from other token-passing standards and later Ethernet-based fieldbuses that offered broader interoperability, lower cost, and easier integration with existing networks. As Ethernet and modern fieldbus technologies matured, IEEE 802.4 Token Bus was gradually superseded, and the standard was eventually withdrawn. Today TokenBus is primarily of historical interest and is encountered mainly in legacy systems or for compatibility studies.