Wika T53 Uživatelský manuál stáhnout pdf (Strana 36)

5.0 Link Active Scheduler (LAS)

5.1 Introduction:

T53 features a LAS function which is only available in Fieldbus Foundation installations.

Please note that the LAS function has not been specified by the Profibus Nutzerorganisation

and is therefore not available in Profibus installations.

5.2 Overview

5.3 Description

All links have one and only one Link Active Scheduler (LAS). The LAS operates as the bus ar

biter for the link. The LAS does the following:

• recognizes and adds new devices to the link.

• removes non-responsive devices from the link.

• distributes Data Link (DL) and Link Scheduling (LS) time on the link. Data Link Time is a

network-wide time periodicallydistributed by the LAS to synchronize all device clocks

on the bus. 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. It is used by system management to synchronize function block execution

with the data transfers scheduled by the LAS.

• polls devices for process loop 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. When a 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 that wins the bid

begins operating as the LAS immediately upon completion of the bidding process. Link mas

ters that do not become the LAS act as basic devices. However, the link masters can act as

LAS backups by monitoring the link 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 con

trolled 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 “Live List.”

Two types of tokens are used by the LAS. A time-critical token, compel data (CD), is sent by

the LAS according to a schedule. A non-time critical token, pass token (PT), is sent by the

LAS to each device in ascending numerical order according to address.

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Wika T53 Uživatelský manuál Strana 36