Link Active Scheduler (Las) - Endress+Hauser PROline Prosonic Flow 93 FOUNDATION Fieldbus Operating Instructions Manual

5 Operation
50
The system architecture of the FOUNDATION Fieldbus can be divided into two sub-
networks:
H1 bus system:
At process-near level, fieldbus devices are connected only via the slower H1 bus
system that is specified as per IEC 61158-2.
The H1 bus system allows simultaneous feed to the 2-wire field devices and data
transfer on the two-wire line.
The following points describe some important characteristics of the H1 bus system:
• All fieldbus devices are powered via the H1 bus. Like the fieldbus devices, the power
supply is connected in parallel to the bus line. Devices requiring external power must
use a separate power supply.
• 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 37 ff.).
• 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 higher-level H2 bus system is realised 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 sub-networks and/or where there are a large number of network
users.
5.3.2

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" (LAS), which controls the sequence
of bus communication centrally. The LAS organises 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. New or removed devices are immediately transferred
to the Live List and sent 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 unscheduled 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
deterministic processes.
PROline Prosonic Flow 93 FOUNDATION Fieldbus
Endress+Hauser