Civil Engineering Reference
In-Depth Information
SAE J1939-71 makes the in-vehicle communication between the different ECUs
possible, no or small (e.g. tuning of ECU addresses) adaptions are needed.
The ECUs are more or less “of the shelf”.
The same situation for passenger car is that every OEM sets up his own network
protocol and all ECUs must be adapted to co-exist in the vehicle.
4.4.1
Structure of SAE J1939
The structure of SAE J1939 is the same as the OSI layers. The following stan-
dards will be focused on in this document: SAE J1939-21, SAE J1939-71 and SAE
J1939-73.
The lowest layers (SAE J1939-11 and SAE J1939-15) are almost similar to ISO
11898, i.e. -15 specifies an unshielded twisted cable and -11 specifies a shielded
twisted cable and the CAN bus speed is currently 250 kbps and the CAN identifier
length is 29 bits.
SAE J1939 is working on extending the CAN bus speed to 500 kbps.
SAE J1939-13 defines the connector, which is a nine-pin round Deutz connector
with two pins for CAN and two pins for SAE J1708 which is the physical layer of
SAE J1587. It is not allowed in the USA to use the SAE J1962 (“ISO” or D-shaped)
connector together with SAE J1939 protocol. This specific requirement does not
exist in Europe and Volvo truck and Volvo bus use SAE J1939 together with the
D-shaped connector.
The usage of J1939 is for controlling vehicle or engine application. SAE J1939-
71 is commonly used in heavy-duty applications for at least powertrain applica-
tions. Some OEMs use it also for complete vehicle applications.
There is an in facto agreement in the USA to use SAE J1939-73 for legislated
OBD diagnostics, and some OEMs use the same protocol as an enhanced protocol
for the complete vehicle and have implemented services for software download and
everything which is needed for workshop fault tracing and repair.
Trucks in the rest of the world usually have adopted an ISO protocol, usually ISO
14230 (“Keyword Protocol (KWP) 2000 on CAN”) or ISO 15765-3 (also known as
ISO 14229-3 or DoCAN).
The ISO protocol does not interfere with J1939-71 which is used for vehicle
control.
It is not allowed to implement both SAE J1939-73 and ISO 15765-4 for legislat-
ed OBD protocol. The reason is that independent scan tools will utilize an algorithm
to detect as to which type of legislated OBD protocol is implemented in the vehicle.
The algorithm is based on scanning through all allowed protocols and the tool stops
when it has detected the first protocol.
It would be too advanced for the tool to try to use two completely different pro-
tocols and combine the data.
