Operating the field indicator
RID16
22
Hauser
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’,
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 field indicator, 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
field indicator when the unit is delivered.
5.3.3
Data transfer
We distinguish between two types of data transfer:
•
Scheduled data transfer (cyclic):
all time-critical process data (i.e. continuous
measurement or actuating signals) are transferred and processed in accordance with a fixed
schedule.
•
Unscheduled data transfer (acyclic):
device parameters that are not time-critical for the
process and diagnosis information are only transferred to the fieldbus when needed. This data
transfer is always carried out in the intervals between timed
communication.
5.3.4
Device ID, addressing
Within the FF network, each fieldbus device is identified by a unique device ID (DEVICE_ID).
The fieldbus host system (LAS) automatically gives the network address for this to the field device.
The network address is the address that the fieldbus currently uses.
The FOUNDATION Fieldbus™ uses addresses between 0 and 255:
•
0 to 15
are reserved.
•
16 to 247
are available for permanent devices. Some host systems may further subdivide this
range. This range is typically shortened for efficiency.
•
248 to 251
are available for devices with no permanent address such as new devices or
decommissioned devices.
•
252 to 255
are available for temporary devices, such as handhelds.
The field device tag name (PD_TAG) is given to the device in question during commissioning (see
27). It remains stored in the device even during a supply voltage failure.
5.3.5
Function blocks
The FOUNDATION Fieldbus™ uses predefined function blocks to describe the functions of a device
and to specify uniform data access. The function blocks implemented in each fieldbus device provide
information on the tasks which a device can accept in the whole of the automation strategy.
