A robust fiber optic active star coupler for the SAE linear token-passing multiplex data bus

The Society of Automotive Engineers (SAE) linear token-passing multiplex data bus draft standard (AS4074.1, version 2.0) calls for two sets of fiber-optic medium characteristics which support a generous power loss budget of about 30 dB. Since there are applications for this bus needing an even greater loss budget one solution is to make the access coupler (the star) an active (powered) network component. A description is given of a simple, low-power, high-reliability active star coupler design meeting the overall requirements of the SAE high-speed data bus (HSDB) standard without changes to the fiber-optic implementation characteristics. The basic active star network is introduced and important quantities defined. Technical issues which influence the active star design are discussed. These include the properties of the protocol which have the greatest effect on the design of the active star, such as the initialization procedure, the system monitoring and control times, the preamble, and the intertransmission gap time. Component options are introduced and a completed design presented. Its performance in a high-speed data-bus environment is characterized and modifications to the standard for best active star performance are suggested.<>