PROFIBUS Overview¶
PROFIBUS Basics
PROFIBUS DP is intended for fast data exchange in the field area. Here, central control units (e.g. PLC/PC) communicate with decentralized field devices like I/O, drives and valves via a fast serial connection. The data exchange with the decentralized modules is mainly performed cyclically.
The communication functions, required for data exchange, are defined by the PROFIBUS DP basic functions in accordance to IEC 61158/61784.
For parameterization, diagnosis and alarm handling during the running cyclic data exchange also non-cyclic communication functions are necessary for intelligent field devices.
ISO/OSI model
The definition of PROFIBUS is based on the experience concerning data transmission collected during long years.
One base is the ISO/OSI model (Open Systems Interconnection Reference Model). It is an open layer model with 7 layers for the communication in information processing systems. The model describes uniformed procedures and rules for the exchange of data.
Fieldbus systems normally use only three of the 7 layers:
| ISO/OSI | Transmitting CPU | Receiving CPU | |||
| Layer 7 | Application layer | Application layer | = | Interface to the application program (CPU) with application oriented commands (read, write) | |
| … | … | … | |||
| Layer 2 | Data-link layer | Data-link layer | = | Access control (to the line), telegram (start, length,..), data security (e.g. CRC=Cyclic Redundancy Code) | |
| Layer 1 | Physical layer | Physical layer | = | Definition of the medium (Twinax, optical fiber, ..), coding (“1”=-4V), transmission speed (baud rate).. | |
| Transmission medium (physical) | |||||
As a result of the ISO/OSI layer model, each layer can be defined separately and (nearly) independent of the other layers.
Indeed, it is possible and common to use conventional cables, but also optical fibers as physical layer for the PROFIBUS DP or have a mixture of both in a single bus configuration.
For the application layer, there are also different versions possible, e.g. PROFIBUS DP-V0, PROFIBUS DP-V1 but also others that are not regarded here.
Typical Field Bus Topologies
Overview of transferred data
* The quantities of bytes/words are defined by the connected device.
** Parameters are transferred during power-up.