CAN Basic Architectures - CAN System Engineering - page 7

bits (LSBs) of the identifier ( Identifier Extension ). The concatenated values of
Base Identifier and Identifier Extension are the logical address and priority of
the message.
The last bit of the Arbitration Field is the RTR bit.
• Control Field = 6 bits:
In contrast to frames with 11-bit identifiers, the Control Field of frames with 29-bit
identifiers does not start with the IDE flag and one reserved bit, but with two
reserved bits, r1 and r0 . The last four bits of the Control Field contain the DLC
for the following Data Field .
Valid values for the DLC are 0…15. The DLC values 8…15 are treated as DLC 8,
i.e. the maximum length of a frame's Data Field is 8 bytes. Reserved bits ( r1 and
r0 ) are transmitted dominant (low); receivers accept both dominant and recessive
values for r1 and r0 .
Old CAN controllers, designed prior to the release of CAN 2.0B, would detect
errors in Extended Frames and would destroy them with Error Frames , but these
CAN controllers are no longer in production. All CAN controllers that can process
Extended Frames are also able to process frames with 11-bit identifiers. Therefore,
it is possible to use both frame formats in the same CAN network. Limited CAN
implementations that can send and receive frames with 11-bit identifiers but ignore
Extended Frames may be used for specific applications.
The IDE bit distinguishes between the frame formats. This bit is recessive in
Extended Frames; hence, an Extended Frame will lose arbitration to another frame
with an 11-bit identifier that is identical to the Extended Frame's Base Identifier .
The Data Frames shown in Fig. 1.1 are the regular messages that distribute net-
work data. Under normal circumstances, such a message is sent on the initiative of
the application program in the transmitting node. Alternatively, CAN also provides
the option that a recipient of a specific piece of information may prompt the sender
to transmit the actual data values. This is done via a Remote Frame . Such a frame
is characterized by its recessive RTR bit and its Data Field—independent of the
DLC—contains no data. A Remote Frame will prompt a sender to transmit its cor-
responding Data Frame. This response can be done automatically by the CAN con-
troller (with “Full CAN” controller) or under the control of the application program
(“Basic CAN” controller). It should be noted that several higher priority messages
may be sent on the CAN bus between the query, i.e. the Remote Frame, and the
answer, i.e. the Data Frame.
Remote Frame may, e.g. be used when a receiver was temporarily inactive, that
is, not participating in bus communication, and needs the current data value before
their sender reaches its next scheduled transmit time.
The structure of a Remote Frame is shown in Fig. 1.2 . The frame structure is
identical in CAN 2.0A and CAN 2.0B except for the length of the Arbitration Field .
A Remote Frame must always be sent with the same DLC as the correspond-
ing Data Frame. If more than one CAN node would simultaneously start Remote
Frame s with the same identifier but different DLCs , these frames would destroy
each other.