The CANopen object dictionary

The CiA organization (CAN in Automation) pursues among other things the goal of creating order and exchangeability between devices of the same type by the standardization of device descriptions. For this purpose so-called CANopen profiles are defined, which conclusively describe the changeable and unchangeable parameters of a device. Such a parameter encompasses at least the following characteristics:

  • An index number – for the unambiguous identification of all parameters.
    The index number is divided into a main index and a subindex in order to mark and arrange associated parameters. The subindex is separated by a colon ":".
    This achieves an arrangement in two levels (logical segments). The main index is always used hexadecimally in the value range 0…65535 (0x0…0xFFFF). The subindex is generally used decimally in the value range 0…255 (0x0…0xFF).
  • An official name - in the form of an understandable, self-descriptive text
  • The access possibility – e.g. whether the parameter can only be read or also written
  • A data type – depending on the parameter this can be of the type Text (string), Number (integer, real), Bool or Byte Field.

The assignment of the index numbers to the parameters is defined in a CANopen profile. In this way all parameters are organized hierarchically as in a table. This table then contains all of the device-specific parameters. It is called the CANopen Object Dictionary (OD).

All the parameters of the TwinCAT EAP device are similarly organized with the help of an object dictionary. In terms of the concept its structure is identical to that of the CANopen OD. The profile for the OD of an EAP device was specified by the EtherCAT Technology Group (ETG) in the specification for the EtherCAT Automation Protocol (ETG 1005, see webpage www.ethercat.org).
This profile is identified by the profile number 5002. It defines the profile type (main profile) and is saved in the Low Word (bits 0-15) of the OD parameter Device Type. The High-Word (bits 16-31) contains the number 1000. It defines the module profile (subprofile). This produces a value of 0x03e8138a (65541002dec) for the Device Type parameter, which is also saved under the Product Code in the Identity Object (index 0x1018:02).

Example of an object in the OD:
The profile of a TwinCAT EAP device is unambiguously identified on the basis of four parameters. These are summarized in a logical segment called Identity, which has the main index 4120 (0x1018). The Vendor ID parameter has the index number 4120:01 (0x1018:01) and the entered value 2 as the identifier of a Beckhoff device.
The logical segment Identity is also designated as object and is represented from the point of view of the user as follows:

The CANopen object dictionary 1:

All the parameters of the Identity object have the property RO (read only), because the parameters should not be changed by the user.

Quite different properties can be described with the aid of the parameters of an Object Dictionary . Examples of such parameters are vendor identifier, version number, process data settings, device name, calibration values, etc. The contents of the OD are required for the commissioning as well as the diagnosis of the EAP device and can be very extensive.

Further Information