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One of the most common network structures is the line structure. Star, tree or mixed
network structures are also possible using connecting components (junction boxes).
The bus connection to the individual fieldbus devices is achieved by means of a
T-connector or via a spur. This has the advantage that individual fieldbus devices can be
connected or disconnected without interrupting the bus or the bus communication.
The number of connected fieldbus devices depends on various factors, such as use in
hazardous areas, length of spur, cable types, current consumption of field devices etc. (see
→
page 8).
If using fieldbus devices in a hazardous area, the H1 bus must be equipped with an
intrinsically safe barrier before the transition to the hazardous area.
A bus terminator is required at each end of the bus segment.
High Speed Ethernet (HSE):
The superior bus system is realized via the High Speed Ethernet (HSE) with a transmission
rate of max. 100 MBit/s. This serves as the 'backbone' (basic network) between various local
subnetworks and/or where there is a large number of network users.
4-3-2. Link Active Scheduler (LAS)
The Foundation Fieldbus™ works according to the 'producer-consumer' relationship.
This provides various advantages.
Data can be directly exchanged between field devices, e.g. a sensor and an actuating valve.
Each bus user ’publishes’ its data on the bus and all the bus users configured accordingly
obtain this data.
Publication of this data is carried out by a ’bus administrator’ known as the ’Link Active
Scheduler’, which controls the sequence of bus communication centrally. The LAS organizes
all the bus activities and sends appropriate commands to the individual field devices.
Other tasks of the LAS are:
Recognition and reporting of newly connected devices.
Reporting the removal of devices no longer communicating with the fieldbus.
Keeping the ’Live List’. This list, in which all the fieldbus users are recorded, is checked by
the LAS regularly. If devices are logged on or logged off, the “Live List” is updated and sent
immediately to all the devices.
Requesting process data from the field devices in accordance with a fixed schedule.
Allocation of send rights (tokens) to devices between the untimed data transfer.
The LAS can be run redundantly, i.e. it exists both in the process control system and in the
field device. If one LAS fails, the other LAS can accurately take over communication. Through
precise timing of the bus communication via the LAS, the FF can run exact processes at
regular intervals.
Note: Fieldbus devices, such as this head transmitter, which can take over the LAS function
in the event of failure of the primary master, are called 'Link Masters'. In contrast, 'Basic
Devices' can only receive signals and send them to the central process control system.
The LAS function is deactivated in this head transmitter when the unit is delivered.