Higher Level Protocols - CAN System Engineering - page 232

Fig. 4.26 SAE J1939-21 PDU and the CN-identifier assignment

the CAN protocol will include bit stuffing and extend the number of bits in the
message.

Some messages are longer than one single CAN frame (8 byte), e.g. VIN which
consist of 17 ASCII characters. When a long message (up to 1785 bytes) should be
transmitted, it is possible to send the message as a segmented message using the
transport protocol function and there are two methods:

4.4.2.1 Transport Protocol (TP)

Method 1: Broadcast Announce Message, TP_BAM TP_BAM: A message with
a global address, which means that all ECUs listen to the message. The message
starts with a Connection Management (CM) message, PGN 00EC00 with a control
byte indicating TP_BAM and then the PGNs with an inter-frame time of minimum
50 ms. This method should not be used if it is not specified in the applicable standard
(i.e. SAE J1939-71, SAE J1939-73 or SAE J1939-03). The main reason is that all
ECUs have to listen to something which may be a message between one ECU and a
scan tool and therefore spend resources on a message which does not concern them.
One of the examples when a TP_BAM could be used is during scan tool initial-
ization before the tool knows which ECUs are installed in the vehicle. It can at that
time broadcast a message in order to identify all that support the service or data.
To identify if a vehicle is utilizing SAE J1939-73 as a legislated OBD protocol,
the DM 5 (Readiness 1) is used (Fig. 4.27 ).

Method 2: Connection Management, TP_CM The other method is called TP_CM:
A message is sent from point to point. The sending ECU sends a CM message indi-
cating Request to Send (RTS). The receiver responds with a Clear To Send (CTS)
with the number of packets (buffer size) it may accept and the sequence number of
the expected packet.
The parameter group, together with the data is then transmitted in several data
transfer messages (DT), wherein the first byte indicates the sequence number in
each case. It is possible to pause the communication and to abort. This method is
the preferred one when it comes to diagnostic communication to an off-board client
(scan tool) (Fig. 4.28 ).