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The Object Dictionary (called "OD" for short) is like a table that holds all network-accessible data.
Each CANopen node must implement its own Object Dictionary.
According to the device type (servo motor, etc) there exists a number of draft recomendations that the device should
implement. It is not mandatory and each vendor has its own specific entries.
The Object Dictionary is organized as a collection of entries, rather like a table.
Each entry has a number called an Index, which is used to access the entry.
The Index is 16 bits in size giving a maximum of 65,536 entries.
Each entry in the Object Dictionary may have up to 256 Subentries,
referenced using an 8-bit value called the Subindex.
Each entry has at least one Subentry.
Not all entries in the Object Dictionary are implemented or used, creating gaps in the table.
For example, the entries with Indexes 0000h - 09FFh are often not implemented,
but the entry with Index 1000h is always implemented.
For example an easy way to scan CANOPEN nodes is to query the entry of each node at Index 1000h.
The rule used is the following: for entries that store only one value, there is only one Subentry at Subindex 00h.
Entries that store more than one value must have a Subentry for each value, and store the number of the highest Subentry at Subindex 00h.
The 65,536 possible Indexes are divided up into sections structuring the Object Dictionary:
|0001h – 001Fh||Standard data types
|0020h – 003Fh||Complex data types
|0040h – 005Fh||Manufacturer specific data types
|0060h – 007Fh||Device profile specific standard data types
|0080h – 009Fh||Device profile specific complex data types
|00A0h – 0FFFh||Reserved for future use
|1000h – 1FFFh||Communication Entries
|2000h – 5FFFh||Manufacturer Specific
|6000h – 9FFFh||Device Profile Parameters
|A000h – FFFFh||Reserved
The Object Dictionary contains several different types of data.
The data may be stored in
The descriptions of the data types used are also stored in the Object Dictionary.
The Object Dictionary stores
- the configuration information for the CANopen communications used by the node;
- any manufacturer specific information;
- various data for device profiles.
CANopen is a communication protocol and device profile specification for embedded systems used in automation.