DEEM CONTROLS Inc.

How Do Absolute Encoders Work?

Can Interface

CAN stands for Controller Area Network and was developed by the company Bosch for applications within the automobile area. In the meantime, CAN has become increasingly used for industrial applications. CAN is a multi-masterable system, i.e. all users can access the bus at any time as long as it is free.
CAN doesn’t operate with addresses, but with message identifiers. Access to the bus is performed according to the CSMA/CA principle (carrier sense multiple access with collision avoidance), i.e. each user listens if the bus is free, and if so, is allowed to send messages.
If two or more users attempt to access the bus simultaneously, the one with the highest priority (lowest identifier) receives the permission to send. Users with lower priority interrupt their data transfer and will access the bus when it is free again.
Messages can be received by every participant. Controlled by an acceptance filter, the participant accepts only messages that are intended for it.

FRABA rotary encoders support two CAN protocols:

CANopen and DeviceNet.

Characteristics - CANopen

Transmission technology
Two-core cable
Baud rate
20 kBaud up to 1 MBaud
Participants
maximum 127
Cable length
30 m for 1 MBaud 5000 m for 20 kBaud
CHARACTERISTIC
CANopen

Transmission technology

Two-core cable

Baud rate

20 kBaud up to 1 MBaud
Participants
maximum 127
Cable length
30 m for 1 MBaud 5000 m
for 20 kBaud
The data communication is done via message telegrams. In general, telegrams can be split in a COB-Identifier and up to 8 following bytes.
The COB-Identifier, which determines the priority of the message, is made from the function code and the node number.
The node number is uniquely assigned to each user. With a FRABA rotary encoder this number is assigned by numerical coded turn switches in the connection cap.
The function code varies according to the type of message transmitted:
  • Administrative messages (LMT, NMT)
  • Service data objects (SDOs)
  • Process data Objects (PDOs)
  • pre-defined messages (synchronization, emergency messages)
PDOs (Process Data Objects) are needed for real time data exchange. Since this messages possess a high priority, the function code and therefore the identifier are low.
SDOs (service data objects) are necessary for the bus node configuration (e.g. transfer of device parameters). Because these message telegrams are tranferred acyclicly (usually only while powering up the network), the priority is low.

FRABA-Encoder with CANopen-Interface

FRABA rotary encoders with CANopen interface support all CANopen functions. The following operating modes can be programmed:
  • Polled mode: The position value is only given upon request.
  • Cyclic Mode: The position value is written cyclically (interval adjustable) to the bus.
  • Sync mode: After receiving a sync message by the host, the encoder answers with the current process value. If a node is not required to answer after each sync message, a parameter sync counter can be programmed to skip a certain number of sync messages before answering again.
  • Change of state mode: The position value is transferred when changing.
Further functions (direction of rotation, resolution, etc.) can be parameterized. FRABA rotary encoders correspond with the class 2 profile for encoder (DSP 406), whereby the characteristics of rotary encoders with CANopen interface are defined. The link to the bus is made by terminal blocks in the connection cap. In additon, the node number and Baud rate are set with turn switches. For configuration and parameterization various software tools are available from different providers. With the help of the provided EDS file (electronic datasheet) simple line-up and programming are possible.

Characteristics - DeviceNet

Transmission technology
Two-core cable
Baud rate
max. 500 kBaud
Participants
maximum 64
Cable length
100 m for 500 kBaud
CHARACTERISTIC
DeviceNet

Transmission technology

Two-core cable

Baud rate

max. 500 kBaud
Participants
maximum 64
Cable length
100 m for 500 kBaud
A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollersand devices to communicate with each other’s applications without a host computer.
It is a message-based protocol, designed originally for multiplex electrical wiring within automobiles to save on copper, but can also be used in many other contexts.
For each device the data in a frame is transmitted sequentially but in such a way that if more than one device transmits at the same time the highest priority device is able to continue while the others back off. Frames are received by all devices, including by the transmitting device.
The DeviceNet protocol is based on the system of connections. In order to exchange information with a device, a connection must first be established. The CAN identifier is used for the characterisation of this connection.

FRABA-Encoder with DeviceNet-Interface

FRABA-Encoder
with DeviceNet-Interface

FRABA rotary encoders with DeviceNet interface support all DeviceNet functions The following operation modes can be programmed:
  • Polled mode: The position value is only given upon request.
  • Cyclic Mode: The position value is written cyclically (interval adjustable) to the bus.
  • Change of state mode: The position value is transferred when changing.
Additional parameters are also programmable such as direction of rotation, resolution and preset value. The adjustment of the node number and the Baud rate takes place in the connection cap using the turn switches.
Easy programming and configuring is possible using the provided EDS file (electronics data sheet) with popular configuration tools.