CANopen Basics Instruction Manual pdf - Kuhnke

CANopen Basics
7.2.1 Bus Allocation by Bit-wise Arbiting
Message A
Message B
Message C
Fast data transfer via a serial bus demands fast bus allocation
mechanisms to decide which one of several parallel
transfer requests is served first. In the case of CAN, the
identifier of a message is the means to provide this
mechanism. The identifier with the lowest number to it has
the highest priority. The priorities are defined by setting
the appropriate binary values at the system design stage;
these values cannot later be changed dynamically.
How bus conflicts are avoided
Conflicts on the bus are avoided by bit-wise arbiting and
the identifiers in that every station monitors the bus signal
level with every bit. The dominant status (0) overwrites the
recessive status (1). All stations transmitting at a recessive
signal level while monitoring the bus at a dominant
level will lose the fight and automatically turn into recipients.
This simple mechanism ensures that the message
with the highest priority will be automatically transferred
across the bus.
Example:
Identifier Data
10 9 8 7 6 5 4 3 2 1 0
Messages A and B will be denied access to the bus because
message C has higher priority (identifier). The relevant
slaves will turn into recipients.
88 E 615 GB
06.02.2008