Profibus Encoder Interface

Introduction

PROFIBUS bus was the first international, open producer-independent standard fieldbus for building, manufacturing and process automation (in accordance with EN 50170). There are three different versions: Profibus FMS, Profibus PA and Profibus DP. Profibus FMS (Fieldbus Message Specification) is appropriate for object-oriented data exchange in the cell and field area. Profibus PA (Process automation) fulfills the request of the process industry and can be used for the intrinsically safe and not intrinsically safe area. The DP version (decentral peripherie) is for fast data exchange in the field of building and manufacturing automation. POSITAL Profibus encoders are ideal for this area.

A profibus system consists of one or more masters and one or more slaves, which are connected by bus cables and bus plugs. A bus segment consists of a maximum of 32 field devices. If more devices are necessary, it is possible to link more bus segments by using repeaters (signal amplifiers). At the end of each bus segment a termination resistor must be used. The number of slaves, which can be operated by a master, is dependent on the internal memory structure of the master. Up to 126 stations can be involved in the maximum configuration of a profibus system. The master is usually realized as a connecting module at the control system or as a pc interface card. Typical slave devices are sensors, actuators, transducers or display elements. The POSITAL Profibus encoder operates as a slave in the profibus system.

Using a software configuration tool, a database is generated which contains the network structure with the necessary configuration and parameter data. The master accesses this database and transmits configuration data to the appropriate users when the profibus system is powered up. After this data is received and stored by the individual users, the system changes to ?data exchange? mode. Profibus operates according to the master slave principle with token passing. The master regulates the bus traffic. The request of input data and the writing of output data between the master & slaves are performed cyclically. If several masters exist, the access right is regulated by the exchange of a token.

Characteristics

POSITAL Encoder with Profibus Interface

The POSITAL encoder operates on the profibus as a slave. Straightforward configuration is possible by the use of the provided GSD file (electronic data sheet). At the beginning of the configuration the address of the device (which identifies the encoder exactly) and the device class are determined. The selected device class determines the specifications of the encoder.

Encoder Classes

The professional bus user organization (PNO) describes obligatory encoder profiles, which are called class 1 and class 2. Absolute rotary encoders class 1 cannot be parameterized, rotary encoders class 2 can be parameterized. Additionally POSITAL Profibus rotary encoders have manufacturer-specific additional functions (e.g. velocity output) that can be selected during the device configuration.

Project Tools

The selection of the device class also determines the length of the input and output data. After the selection of the device class is performed, the appropriate parameters (e.g. resolution, direction of rotation, software limit switch, etc..) are saved in a database and transferred to the rotary encoder when starting the system. Data can be read from the rotary encoder (e.g. position value) or written to the encoder (e.g. preset value) using the input and output addresses determined in the configuration. The Baud rate is also determined in the configuration and is detected automatically by RABA rotary encoders. Further adjustments are not necessary with this system.