Technical data

PROFIBUS networks
1.2 Basics of the PROFIBUS network
PROFIBUS Network Manual
18 System Manual, Edition 04/2009, C79000-G8976-C124-03
DP protocol
At the field level, protocols for PROFIBUS with large numbers of services or complicated
data processing are unsuitable because the required bus cycle time and reaction time
cannot be achieved.
To be able to cover the field level within the automation hierarchy, PROFIBUS DP
(distributed peripheral I/O) was developed. The essential characteristic of PROFIBUS DP is
that the user data is represented in the form of a cyclic data image. Here, object-oriented
interfaces as used in FMS or the S7 protocol are completely avoided. The principle of
PROFIBUS DP communication is a master-slave system. A master polls one or more slaves
cyclically.
Instead of the normal user interface, at layer 7 (ISO-OSI reference model) of the DP
protocol, there is a user interface in the form of a standard application that along with the
DDLM (Direct Data Link Mapper) interfaces directly with layer 2 (ISO-OSI reference model),
in other words with FDL.
There are two types of DP masters:
● Class 1 master: Controls the process cyclically
● Class 2 master: Device parameter assignment and diagnostics
Benefits:
● Very fast communications protocol because very hardware oriented
● Can be used with third-party systems
FMS protocol
In the original form of the PROFIBUS specification, not only the FDL protocol but also the
FMS protocol was specified. The aim of this protocol was to be able to include more complex
hierarchical systems alongside the field devices.
To achieve this, part of the MMS (Manufacturing Message Specification) that originated from
the MAP communications model was met in addition to field device interfacing. The complete
model resulted in the Fieldbus Message Specification (FMS).
Within PROFIBUS, levels 3 to 6 are not implemented. The user layer is layer 2; for layer 7,
the Lower Layer Interface (LLI) was developed for the FMS protocol. Functions of the non-
existent layers such as connection establishment and termination and connection monitoring
are implemented in this LLI for the FMS protocol.
The FMS protocol is object-oriented. All transferred data is transferred in the form of non-
proprietary, standardized communications objects. Each object is accessed via its index or
name.
Benefits:
● Acknowledged data transfer
● Can be used flexibly with third-party systems
● Access to individual variables or structure elements is possible
● Linking to slaves and masters possible