FOUNDATION Fieldbus is an open standard for networking input, output, and process control devices that, when configured, can run independently of a computer system. Fieldbus can operate independently of a computer system because devices called Link Masters (such as the FP-3000) have processing capability and are capable of controlling the bus. There are also basic devices that cannot control the bus, but provide input, output, and/or additional functionality, such as PID or expression block execution.
You can have multiple Link Masters on the bus; however, only one Link Master can control the bus at a time. The device that is currently controlling the bus called the Link Active Scheduler (LAS). This device has the schedule for all the devices and sends commands to other devices, telling them when to broadcast data. The LAS also publishes time information and allocates time to devices to allow them to broadcast unscheduled messages, such as alarms.