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 EN 50170.
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
PROFIBUS® DP is standardized under IEC 61158.
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:sup:® 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:sup:® DP-V0, PROFIBUS® DP-V1 but also others that are not regarded here.
Typical Field Bus Topologies
Typical field bus topologies
Overview of transferred data
Overview of transferred data
* The quantities of bytes/words are defined by the connected device.
** Block parameters are transferred during power-up.
The PDP22 (PROFIBUS:sup:® DP-V1) allows to suppress the block parameter transfer (executed e.g. during power-up) setting the appropriate parameter. This parameter is not sent to the device and cannot be set via the device.
In the Control Builder AC500 used for the CPU AC500, the parameter is ‘Ignore Block Parameters’ or ‘Use Block Parameters’ respectively in the PDP22 parameter part.