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CANopen devices must maintain an object dictionary, which is used for configuration and communication with the device. An entry in the object dictionary is defined by:

 Index, the 16-bit address of the object in the dictionary.

 Object name (Object Type/Size), a symbolic type of the object in the entry, such as an array, record, or simple variable.

 Name, a string describing the entry.

 Type, this gives the datatype of the variable (or the datatype of all variables of an array).

 Attribute, which gives information on the access rights for this entry, this can be read/write, read-only, or write-only.

 The Mandatory/Optional field (M/O) defines whether a device conforming to the device specification has to implement this object or not.

The basic data types for object dictionary values such as Boolean, integers, and floats are defined in the standard (their size in bits is optionally stored in the related type definition, index range 0x0001–0x001F), as well as composite data types such as strings, arrays, and records (defined in index range 0x0040–0x025F). The composite data types can be subindexed with an 8-bit index; the value in subindex 0 of an array or record indicates the number of elements in the data structure, and is of type UNSIGNED8.