Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 68)

Publication 1757-UM006A-EN-P - May 2002
2-24 Integrating Fieldbus into Rockwell Automation Logix System
Link and Block Schedules Link Active Scheduler (LAS)
All links must have a Link Active Scheduler (LAS). The LAS operates at
the data link layer as the bus arbiter for the link. It dynamically
provides the following functions.
• Recognizes and adds new devices to the link.
• Removes non-responsive devices from the link.
• Distributes Data Link and Link Scheduling time on the link. The
data link layer synchronizes the network-wide Data Link Time.
Link scheduling time is a link specific time represented as an
offset from Data Link Time. It is used to indicate when the LAS
on each link begins and repeats its schedule. System
Management uses it to synchronize function block execution
with the data transfers scheduled by the LAS.
• Polls devices for buffered data at scheduled transmission times.
• Distributes a priority-driven token to devices between scheduled
transmissions.
Any device on the link may become the LAS as long as it is capable.
The devices that are capable of becoming the LAS are called Link
Master devices. All other devices are referred to as Basic devices.
The FIM is Link Master capable and supports both a primary and a
backup link schedules. It is designated as the primary Link Master.
Upon startup or failure of the existing LAS, the Link Master devices on
the link bid to become the LAS. The Link Master that wins the bid
begins operating as the LAS immediately upon completion of the
bidding process. Link Masters that do not become the LAS act as basic
devices when viewed by the LAS. They also act as LAS backups by
monitoring the link for failure of the LAS, and by bidding to become
the LAS when a LAS failure is detected.
ATTENTION
!
If a LAS is too large to fit in the active Link Master
capable device, the user must reconfigure the device
to become a Basic one through Control Builder, and
restart the device to initiate the change.