Extended Format Can Message - Omron CJ1W-CORT21 Operation Manual

1 Features and System Configuration
1-2-9

Extended Format CAN Message

 29-bit Identifier
The Society of Automotive Engineers (SAE) "Truck and Bus" sub-committee standardized signals
and messages as well as data transmission protocols for various data rates. It became apparent that
standardization of this kind is easier to implement when a longer identification field is available.
To support these efforts, the CAN protocol was extended by the introduction of a 29-bit identifier.
This identifier is made up of the existing 11-bit identifier (base ID) and an 18-bit extension (ID exten-
sion). Thus the CAN protocol allows the use of two message formats: StandardCAN (Version 2.0A)
and ExtendedCAN (Version 2.0B). As the two formats must coexist on one bus, it is determined
which message has higher priority on the bus in the case of bus access collisions with differing for-
mats and the same base identifier. The message in standard format always has priority over the
message in extended format.
 Extended Format
CAN controllers which support the messages in extended format can also send and receive mes-
sages in standard format. When CAN controllers which only cover the standard format
(Version 2.0A) are used on one network, then only messages in standard format can be transmitted
on the entire network (messages in extended format would be misunderstood). However there are
CAN controllers which only support standard format but recognize messages in extended format
and ignore them (Version 2.0B passive).
The distinction between standard format and extended format is made using the IDE bit (Identifier
Extension Bit) which is transmitted as dominant in the case of a frame in standard format. For
frames in extended format it is recessive.
 RTR Bit
The RTR bit is transmitted dominant or recessive depending on whether data are being transmitted
or whether a specific message is being requested from a unit. In place of the RTR bit in standard for-
mat the SRR (substitute remote request) bit is transmitted for frames with extended ID. The SRR bit
is always transmitted as recessive, to ensure that in the case of arbitration the standard frame
always has priority bus allocation over an extended frame when both messages have the same
base identifier.
 IDE Bit
Unlike the standard format, in the extended format the IDE bit is followed by the 18-bit ID extension,
the RTR bit and a reserved bit (r1).
All the following fields are identical with standard format. Conformity between the two formats is
ensured by the fact that the CAN controllers which support the extended format can also communi-
cate in standard format.
1-18
Arbitration field
S S I
O 11 bit R D 18 bit
F IDENTIFIER R E IDENTIFIER
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
Control Data Field CRC Field
Field
R r r
T f 0 DLC 0 - 8 bytes 15 bit CRC
R
Ack End of Int Bus idle
Field Frame