Token Bus Topology: Working, Token Passing, Frame Format, Differences & Limitations

Introduction

A Token Bus network uses a virtual ring over a physical bus (coaxial cable) to implement the Token Ring access method. A token is passed between network nodes, and only the node holding the token is allowed to transmit data. If a node has no data to send, it simply passes the token to its logical neighbor in the virtual ring. For this reason, each node must know the address of its adjacent node in the logical sequence.

Ethernet was originally considered unsuitable for manufacturing automation because it offered no guaranteed maximum waiting time for any device. Token Bus was therefore developed to combine the deterministic access control of Token Ring with the simpler physical structure of a bus network.

Token Bus is defined under the IEEE 802.4 standard and was widely used in industrial environments. General Motors used it as part of the Manufacturing Automation Protocol (MAP). Although based on Token Ring concepts, Token Bus does not form a physical ring—its endpoints do not meet.

A modified version of Token Bus was later proposed for Manufacturing Automation Systems and flexible manufacturing systems (FMS) to guarantee packet delay and reliable transmission.

Token Passing Mechanism in Token Bus

A token is a small control frame that circulates among the stations in a logical order. When a station receives the token:

• If it has data to send, it captures the token, transmits the data, and then forwards the token to the next station.
• If it has no data to transmit, it simply passes the token onwards.

Although stations are physically connected through coaxial cable, they function as though arranged in a logical ring. Access to the bus is controlled by the circulating token. This ensures deterministic communication—important for industrial applications.

General Motors’ MAP system used Token Bus because of its predictable timing. A modified Token Bus protocol was also used in flexible manufacturing systems to ensure reliable packet delivery.

Token Bus Frame Format

The major fields in a Token Bus frame are listed below:

• Preamble: A one-byte field used to synchronize bits.
• Start Delimiter: A one-byte field that indicates the beginning of a frame.
• Frame Control: Specifies frame type (data or control). One byte.
• Destination Address: Contains the destination address (2 to 6 bytes).
• Source Address: Contains the source address (2 to 6 bytes).
• Data: Up to 8182 bytes for 2-byte addressing, and up to 8174 bytes for 6-byte addressing.
• Checksum: A four-byte error-checking field.

Differences Between Token Ring and Token Bus

Limitations of Token Bus

• If the bus cable fails, all devices beyond the failure point lose connectivity.
• Adding new stations is complicated. Poorly connected devices may degrade communication for all nodes beyond them.
• Network reconfiguration (addition or removal of stations) requires special signaling, which adds complexity.