Link Active Scheduler (Las); Fieldbus Device List - Emerson AMS Trex User Manual

Field Communicator application
Related information
3.18.4

Link Active Scheduler (LAS)

All fieldbus segments have only one Link Active Scheduler (LAS). The LAS operates as the
bus arbiter for the segment. The Trex unit is the last node to become the LAS on a fieldbus
segment.
The devices that can be the LAS are called link master devices. All other devices are called
basic devices. Use the Trex unit to set a device to be a link master or basic device.
When a fieldbus segment first starts up, or upon failure of the existing LAS, the link master
devices on the segment bid to become the LAS. The link master with the lowest address
begins operating as the LAS immediately after completing the bidding process. Link
masters that do not become the LAS can act as LAS backups by monitoring the fieldbus
segment for failure of the LAS and then bidding to become the LAS when a LAS failure is
detected.
Only one device can communicate at a time. Permission to communicate on the bus is
controlled by a centralized token passed between devices by the LAS. Only the device with
the token can communicate. The LAS maintains a list of all devices that need access to the
bus. This list is called the Device List.
Although the Trex unit can be the LAS, it does not maintain the communication schedule
and should not be thought of as a backup LAS. When the Trex unit is connected to a live
fieldbus segment, it joins the fieldbus segment, but not as the LAS. The Trex unit can view
and edit device parameters, but the LAS controls the segment.
The device acting as the LAS is labeled on the Device List screen.
3.18.5

Fieldbus Device List

The Device List displays all devices that the Trex unit has detected. Each device is listed by
its tag and address. The last four numbers of the Trex serial number are displayed to
identify the Trex unit. The Device List also identifies the host on the fieldbus segment and
which device is the LAS.
Tap a device listed on the screen to connect to it. If a device is not listed, tap the My Device
Not Found button. This option expands the following settings:
•
•
100
FOUNDATION fieldbus communication terminals
Power and connect to a FOUNDATION fieldbus device
Connect to an externally-powered FOUNDATION fieldbus device
Polling addresses - By default, the Trex unit polls addresses 0-48 and 232-255. This
range is then expanded to include all valid fieldbus addresses.
Slot time - The slot time is increased to enable the Trex unit to find older
FOUNDATION fieldbus devices on the fieldbus segment.
User Guide