JVL MAC050 User Manual page 116

4.4 Expansion Modules MAC00-FC2/FC4
Multi-Master capability:
Bus access rights are not issued by a mean-level control unit (bus master) per network.
Each participant can rather 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 arbitration
process allocates each participant the bus access right in line with the priority of the mes-
sage they want to send at that particular moment. Each participant can therefore com-
municate 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 transmission
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 CAN protocol, it is possible that several par-
ticipants want to occupy the bus at the same time. In other random bus access routines,
this causes the destruction of the suppressed messages. In order to solve such a bus ac-
cess conflict, a repeated occupation of the bus is required using an appropriate triggering
strategy. The CAN 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 contents.
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.
4.4.6 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 the
figure below:
TT1093GB
The two bit fields "Header" and "Data" form the simplified CANopen® message. The
11-bit Header are also designated as the identifier or as the COB-ID (Communication
Object identifier).
JVL uses 11-bit format type CAN A, but not 29-bit format type CAN B.
The COB-ID carries out two task for the controller communications object.
- Bus arbitration: Specification of transmission priorities.
- Identification of communications objects.
The COB-ID comprising two sections:
- Function code, 4 bit in size (0....15)
- Node address (Node ID), 7 bit in size (0....127). See section 4.3.12.
112 JVL Industri Elektronik A/S - User Manual - Integrated Servo Motors MAC050 - 800
0....8 Byte
0
11 bit
Header
Data frame
1 2 3 4 6 7
5