Civil Engineering Reference
In-Depth Information
Fig. 4.27
J1939 trans-
port protocol—broadcast
announce message
Request: Normal in-vehicle control data are sent periodically on the bus but
sometimes some data are needed and they are not usually sent on the bus. It is pos-
sible to request the data (or PGN) in those situations. One example is VIN which is
used to identify the vehicle by a scan tool. The VIN is 17 characters and there is no
need to send it periodically, so the scan tool needs to request the data from the ECU
which has the information.
Data: The ECU either sends a negative acknowledge, NACK, if it does not have
the data or respond to data. If one data element is not used in the PGN, then the bits
for that suspect parameter number shall be set to 1.
The response time is 200 ms but the requesting equipment needs to wait up to
1.25 s before it times out. The main reason is that bridges (gateways or routers) can
delay the message.
4.4.3
SAE J1939-31—Network Layer
The SAE J1939-31 network layer standard defines how a complete network should
be designed. The standard describes gateway and router functionalities (Fig.
4.29
).
Gateways can be within the vehicle to isolate different buses from each other
(e.g. one private network for brake system, another network for the cab controller, a
third for powertrain) or to act as bridges between, e.g. a tractor and a trailer.
4.4.4
SAE J1939-71 71—Vehicle Application Layer
SAE J1939 has relationship to SAE J1587 which is usually implemented on SAE
J1708 bus.
J1939-71 defines signals for normal communication, point-to-point in-vehicle
communication. The signals are defined as SPNs.
