Beckhoff BK3000 User Manual download pdf (Page 56)

Appendix

56 BK3xxx/LC3100

Using bus couplers as combislaves carries a number of advantages for the

user:

It reduces the variety of different devices involved, because the identical

device can be used flexibly, either for fast cyclic data transfer with

PROFIBUS DP or with the more powerful PROFIBUS FMS services. For

example, you can use the FMS services for parametrization when the

installation is initially set up, when speed is not such a critical factor, and

the fast DP functions for the cyclic transfer of user data in the operating

phase of a controller. Such possible combinations offer manifold ways of

utilizing these devices.

This combined operation is possible because both versions of the protocol

use the same transfer and bus access procedures (layers 1/2). The various

application functions are separated by the different service access points of

layer 2. The bus coupler automatically recognizes the appropriate

processing mode.

PROFIBUS FMS

(only BK3000 and BK3100)

PROFIBUS FMS makes it possible for automation devices to communicate

with one another and with the intelligent field devices. The available

functionality is more important here than a fast system response time.

Many applications carry out predominantly acyclic data exchanges in

response to requests from the application process. Beckhoff bus couplers

can operate both as DP slaves and FMS slaves.

PROFIBUS LAYER 7

(application layer)

Layer 7 of the ISO/OSI reference model provides useful communication

services for the user. These application services permit efficient and open

data transfer between application processes. The PROFIBUS application

layer is specified in DIN 19 245 part 2 and consists of:

- Fieldbus Message Specification (FMS) and

- Lower Layer Interface (LLI).

FMS describes the communications objects, application services and the

resulting models from the point of view of the communications partner. The

LLI is used to adapt the application functions to the manifold features of the

PROFIBUS layer 2.

PROFIBUS

communication model

An application process includes all the programs, resources and tasks

which do not belong to any of the communication layers. The PROFIBUS

communication model makes it possible to use communication

relationships to combine distributed application processes into one overall

process.

That part of an application process in a field device which is accessible via

communications is described as a virtual field device (VFD).

In the PROFIBUS communication model, the mapping of the functions of

the VFD onto the actual device is carried out by the application layer

interface (ALI).

Communications objects

and object directory (OD)

All the communications objects belonging to a PROFIBUS subscriber are

recorded in his local object directory. For simple devices, the object

directory may be predefined. For complex devices, the object directory is

planned and loaded into the device locally or remotely. The OD contains

descriptions, structures and data types, and also the correlation of the

internal device addresses of the communications objects with their

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