How Do Absolute Encoders Work?

Field bus interface

Since the beginning of the 90’s bus systems have been increasingly used; in particular, open field bus systems such as Profibus, Interbus and CANbus. Unlike conventional techniques these systems are economically interesting. Field buses are not only economically favorable, they also represent a new technology which opens another dimension for the planning of system concepts incorporating decentralized solutions.
Field buses enable the specific communication of automation components – bus nodes – among themselves.

Data exchange

Different bus systems use different principles to transfer data. The most important ones are explained in the following paragraphs:

Master Slave principle with token passing:

The data communication is controlled by the master. Slaves at the bus answer only on request of the masters. Every master has a determined time to exchange data with the slaves. The bus cycle time is therefore calculable. If several masters are available, the bus access right is regulated by exchanging tokens. The master which receives the token, possesses the exclusive right to access the bus.

Priority-controlled data communication:

With this method each user can transmit data at each point in time. To avoid collisions, or to resolve created collisions, there must be mechanisms responsible for arbitration. For example, CSMA/CA procedure (carrier sense multiple access with collision avoidance) prevents the emergence of a collision by simultaneous transmitting, the CSMA/CD PROCEDURE (Carrier Sense Multiple Access with Collision Detection) resolves developed collisions.

Shift register with sum framework protocol:

The bus master transmits the output data in each cycle to all slaves and receives in response the input data of all slaves. The small data range of this procedure becomes balanced by a high log efficiency. The bus cycle time is calculable.

Delegated token:

A central bus arbiter regulates the data communication. It distributes the token, according to certain algorithms, to the individual bus users. If a user is in the possession of the token, it can transmit messages. Subsequently, it returns the tokens to the arbiter.

Selection of the bus system

The selection of a bus system depends very strongly on the application. Individual systems are optimized for the main application. Therefore a universal bus system is not possible. The following technical criteria can assist for the selection of a bus system:
  • availability of the total system
  • test and installation supports
  • diagnostic possibilities
  • protected data communication
  • response time
  • availability of the field bus
  • components
Fraba encoders are available with all common field busses.