Siemens SIMOVERT MASTERDRIVES Manual page 519

Communication / CBC CANopen Communication Board
8.5-4
A central element of the CANopen standard is the description of device
functionality using an "object directory" (OD).
The object directory is divided into two areas, one containing general
data about the device such as identification, manufacturer name, etc.,
plus communication parameters and a second area containing a
description of the device functionality.
An entry ("object") in the object directory is identified by a 16-bit index
and an 8-bit subindex.
The entries in the object directory make the "application objects" of a
device, such as input and output signals, device parameters, device
functions or network variables, accessible in standardized form via the
network.
In a similar manner to other field bus systems, CANopen also uses two
basic data transmission mechanisms, i.e. high-speed exchange of short
process data via so-called "Process Data Objects" (PDOs) and access
to entries in the object directory via so-called "Service Data Objects"
(SDOs). The primary purpose of the latter is to transfer parameters
while equipment is being configured and, in general, to transmit long
data areas. Process data objects are generally transferred in event-
oriented form, cyclically or - on request - as broadcast objects without
additional protocol overhead.
A total of 8 bytes of data can be transmitted in one PDO. The
transmission and receipt of PDOs can be synchronized throughout the
network ("synchronous PDOs") using a synchronization message. The
assignment between application objects and a particular PDO
(transmission object) can be configured via a structure definition ("PDO
mapping") in the OD. Assignments can thus be adapted to meet the
requirements of a particular application.
SDOs are transmitted as a confirmed data transfer, with two CAN
objects per transmission, in the form of a peer-to-peer connection
between two network nodes. The relevant object directory entry is
addressed through specification of the index and subindex of the OD
entry. Messages with a total length of 5 bytes can be transferred.
Transferring SDO messages involves an additional overhead.
Standardized, higher-priority, event-oriented alarm messages
("Emergency_Messages") are provided to signal device faults.
The functionality required for the preparation and coordinated starting
of a distributed automation system complies with the mechanisms
defined by the CAL Network Management (NMT) specification. The
principle of "Node Guarding" underlying the cyclical node monitoring
functions are also compliant with NMT.
CAN message identifiers can be assigned to PDOs and SDOs through
the direct entry of identifiers in the data structures of the object directory
or, for simple system structures, through the use of predefined
identifiers.
6SE7087-6QX70 (Version AD)
Compendium Motion Control
02.2004
Siemens AG
SIMOVERT MASTERDRIVES