How Do Absolute Encoders Work?

Interbus interface

The INTERBUS was developed by the company Phoenix Contact. The specification has been popular since 1987, and INTERBUS components are available from over 200 manufacturers. INTERBUS is a fast, universal and open sensor/actuator bus system with one master and several slaves. Data transmission rate and expansion of the bus are independent of one another. The gross data transmission rate is 500kBit/s, the net data transmission rate is 300kBit/s. For special applications with fiber-optic cable data transmission rates of 2Mbit/s are possible. The number of users is limited to 512.


An INTERBUS system conforms to a ring structure. A compact strand following one direction in the system is used for the bus connection. Beginning at the master (PLC or IPC) the bus system connects the respective control or computer systems to the peripheral input and output modules. The main line of the system is called the remote bus and bridges distances up to 12.8 km between peripheral stations. From the remote bus, branch lines are possible. These branches can be either be an installation remote bus or a local bus.
The data transfer is done using the “shift register with sum framework protocol” (in a data cycle all data is shifted through the ring).


Transmission technology:
Baud rates:
Cable length:

FRABA Encoder with INTERBUS-Interface

The absolute rotary encoder is a remote bus user. The individual users are connected by an installation remote bus cable. This cable carries both the bus line coming from the master and the return line. The connection between the rotary encoder and the bus is made by two 9 pin connectors (male and female). An address assignment is not necessary, since the address of the individual users is given by their physical position on the bus. Projecting and parameterization can for example be done with the INTERBUS CMD software or with PC Works.

Encoder profiles

Three profiles are regulated by the user group ENCOM to ensure smooth data transfer between terminals of different manufacturers:

FRABA rotary encoders can be delivered in K1, K2 and K3.


To connect single sensors and actuators in an economical way, a transmission technology adapted for various common operating conditions was developed. This is called INTERBUS loop.
The INTERBUS loop connects terminals to a ring with a simple two-core unshielded cable. Using these two cores, data information and voltage supply are delivered simultaneously. The data communication takes place in the form of load independent current signals. By this method the INTERBUS loop becomes so interference-proof that a shielded cable is not necessary. The coupling of the INTERBUS Loop to the INTERBUS remote bus is made by a special bus clamp. 63 INTERBUS loop participants can be connected per bus clamp.
The successor of the INTERBUS loop, the INTERBUS loop 2, contains an integrated report and diagnostic manager and enables a larger distance between the users in the loop. The max. distance between the individual terminals is 20 m, the max. loop length 200 m.


For applications demanding high noise immunity or high data transmission rates, fiber-optic cables are available as an alternative to the conventional transmitting media.
The SUPI 3 OPC (Optical Protocol Chip) is used for these demands. It enables a distance diagnosis and optical power adjustment for LWL transmitters. The fibre optical cable can be easily connected to an existing INTERBUS network with a bus clamp.
Advantages of this system are high noise immunity and also data transmission rates of up to 2 MBit/s.