CAN-8123/CAN-8223/CAN-8423 user’s manual (Revision 3.00, May/22/2009) ------
38
Inhibit Time
Because of the arbitration mechanism of the CAN bus, the CANopen
communication object ID in small size has a higher transmission priority than
the bigger one. For example, there are two nodes on the CAN bus, the one
needs to transmit the CAN message with the COB-ID 0x181, and the other has
to transmit the message with COB-ID 0x182. When these two nodes transmit
the CAN message to the CAN bus simultaneously, only the message
containing COB-ID 0x181 can be successfully sent to the CAN bus because of
the higher transmission priority. So the message with COB-ID 0x182 will be
held to transmit until the message with COB-ID 0x181 is successfully
transmitted. This arbitration mechanism can guarantee the successful
transmission for one node when a transmission conflict occurs.
However, if the message with COB-ID 0x181 is continually transmitted,
the message with COB-ID 0x182 will be postponed to be transmitted. In order
to avoid the occupation of the transmission privilege by the message with the
lower COB-ID, the inhibit time parameters for each of the PDO objects are
supported to define a minimum time interval between each PDO message
transmission, which has a multiple of 100us. During this time interval, the PDO
message will be inhibited from transmission.
Event Timer
This parameter setting on the event timer is only used for TxPDO. If the
parameter of the event timer is not equal to 0 under the transmission type in
asynchronous mode, the expiration of this time value can be just considered to
be an event. This event will cause the TxPDO message transmission. The
event timer parameter is defined as a multiple of 1ms.
PDO Mapping Objects
The PDO mapping objects are provided to the interface which is for PDO
messages and real I/O data in the CANopen device. They define the meanings
for each byte in the PDO message, and may be changed by using a SDO
message. All of the PDO mapping objects are arranged in the Communication
Profile Area. In the CANopen spec (see DS 401), RxPDO and TxPDO default
mapping objects will specify something as follows:
