JVL MIS17 Series User Manual page 270

11.1 General information about CANopen
Multi-Master capability:
Bus access rights are not issued by a mean-level control unit (bus master) per network.
Instead, each network node can start to send a message with equal rights as soon as the
bus has become free. If several participants access the bus at the same time, an arbitra-
tion process allocates each participant the bus access right in line with the priority of the
message they want to send at that particular moment. Each participant can therefore
communicate directly with every other participant. As the transmission of a message can
be initiated by the message source itself, then in the case of event-controlled transmis-
sion of messages, the bus is only occupied when a new message is on-hand.
No-loss bus arbitration:
As the bus is accessed at random under the CANopen protocol, it is possible that several
participants try to occupy the bus at the same time. In other random bus access routines,
this causes the destruction of suppressed messages. In order to solve such a bus access
conflict, a repeated occupation of the bus is required using an appropriate triggering
strategy. The CANopen protocol therefore deploys a routine to ensure that the message
with the highest priority at any given time is sent without any destruction of message con-
tents.
Short block length:
The maximum data length of a CAN message is limited to 8 bytes. This data length is usu-
ally sufficient to transmit the information occurring in the lowest field area in a CAN mes-
sage.
11.1.5 Header
A CAN message transmits the communications object and a variety of management and
control information. The management and control information bits are used to ensure
error-free data transmission, and are automatically removed from the received message
and inserted before a message is sent. A simplified CANopen message could be as in the
figure below:
TT1093GB
The two bit fields "Header" and "Data" form the simplified CANopen message. The 11-
bit Header is also designated as the identifier or as the COB-ID (Communication Object
identifier).
270 JVL Industri Elektronik A/S - User Manual - Integrated Stepper Motors MIS17x, 23x, 34x, 43x
0....8 Byte
0
11 bit
Header
Data frame
1 2 3 4 6 7
5