Installation Manual

Electrical & Electronic Application And Installation Manual
Production Release Version 1.0
Page 235
• The bus is linear and should be terminated with 120Ohm resistors at
either end. It is a common mistake to use one 60Ohm resistor instead
of two 120Ohm resistors. This does not work correctly however.
• Maximum bus length is 40m.
• The terminating resistors should not be contained in network nodes.
• Network nodes are connected to the bus via stubs of maximum
recommended length 1m.
18.1.3 Network Layer
• J1939 recommends a bit sample point of 87 percent. This relatively
late sampling point, which gives the best immunity to noise and
propagation delay. It does restrict the size of the software jump width
(SJW), however.
• All nodes must have the same bit timing.
• Accurate bit timing is essential (4ms +/- 0.2 percent).
• It is recommended that the average busload is not greater than 40
percent.
• Hardware filtering (masking) of CAN messages should be used under
high busload limit demands on processors.
• The engine ECM always assumes a fixed address zero. It will not
change its address in the arbitration process described in J1939-81.
• The multi 7 packet protocol (described in J1939-21) is used for sending
messages with more than eight bytes of data. In the machine
application this will be used principally for the diagnostic messages
DM1 and DM2.
• Information may be broadcast or requested at regular intervals. For
example, the engine will broadcast its ‘current speed’ every 20ms but it
will only send ‘hours run’ information if another node requests it.
18.1.4 Application Layer
• The messages (PGN’s) supported by the engine ECM are only a
subset of the messages described in J1939-71 and J1939-73.
• Some PGN’s maybe partially supported i.e. only those bytes for which
the ECM has valid data will be supported.
• Unsupported data bytes are generally sent as FF (hex) and incorrect or
invalid information is sent as FE.