Omron C200HW-CORT21-V1, Operation Manual

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Communication
Section 1-3
16
1-3-5 Predefined Connection Set
CAN message frame
The figure below shows the format of a CAN message frame. Only the most
important parts of the frame are displayed: the 11-bit identifier and the data
field. The other fields are irrelevant for the understanding of the
communication protocol.
11-bit Identifier (ID) The 11-bit Identifier indicates the type of message and determines the
priority of the message. The lower the value of the identifier, the higher the
priority.
Message priority
Message priority is only of importance when two nodes try to transmit at the
same time. In that case, the device that transmits with the lowest identifier
value gains the control over the bus.
Message filtering
PDO linking
When a message is transmitted over the network, each node will receive this
message, but a node can be configured to only process specific messages.
This message filtering is based on the value of the identifier.
In CANopen, a node will only process a message from another node when
the identifier is known in advance, i.e. the node has been configured to filter
the identifier. To achieve this, the ID of an RPDO of the receiving node must
have the same value as the ID of a TPDO of the transmitter (see also section
1-3-1); the PDOs of the 2 devices are 'linked'.
In CANopen, the Identifier of a communication object, like TPDO or RPDO, is
called Communication Object ID or COB-ID.
To reduce the effort of so called ‘PDO linking’ for simple networks, CANopen
has defined a mandatory default identifier allocation scheme: the Predefined
Connection Set. These predefined identifiers are available in the Pre-
Operational state (if no modifications have been stored).
Predefined connection set The predefined identifier scheme of CANopen uses a 4-bit function part
(Function code) and a 7-bit Node-ID part to form the 11-bit identifier.