Question on J1939 CAN Bus

I'm doing self-study on the J1939 CAN bus, and I fairly sure that I
have a reasonable grasp on the fundamentals. Most of what I'm working
with deals with the physical layer, the J1939/11. What I'm struggling
with has to do with the interfacing of J1939 compliant devices, such
as instrumentation and gauges, that are added as nodes on the network.
Considering that the network and backbone are designed to accept a
gauge, e.g. J1939 compliant tachometer, can the gauge simply be added
to the network without any other considerations? In other words, the
tachometer will simply read the stream of messages emanating from
controllers on the network and only intercept those messages with
information relative to the tach, or does the Master Controller need
to know that there is a tach in the network in order to send data that
the tach will intercept? I'm trying to understand and determine if
the J1939 compliant gauge has a unique node ID that the Controller
needs to know exists, in order to send the correct message components.
Any insight on this would be greatly appreciated. This has been a
difficult subject to grapple with.

 

On Thursday, March 18, 2010 10:18:47 AM UTC-5, (unknown) wrote:

I'm doing self-study on the J1939 CAN bus, and I fairly sure that I
have a reasonable grasp on the fundamentals. Most of what I'm working
with deals with the physical layer, the J1939/11. What I'm struggling
with has to do with the interfacing of J1939 compliant devices, such
as instrumentation and gauges, that are added as nodes on the network.
Considering that the network and backbone are designed to accept a
gauge, e.g. J1939 compliant tachometer, can the gauge simply be added
to the network without any other considerations? In other words, the
tachometer will simply read the stream of messages emanating from
controllers on the network and only intercept those messages with
information relative to the tach, or does the Master Controller need
to know that there is a tach in the network in order to send data that
the tach will intercept? I'm trying to understand and determine if
the J1939 compliant gauge has a unique node ID that the Controller
needs to know exists, in order to send the correct message components.
Any insight on this would be greatly appreciated. This has been a
difficult subject to grapple with.

Many J1939 messages can be transmitted in either of two ways: Specific or Broadcast. Specific messages contain a destination network address indicating the message is intended to be read by a single node; Broadcast messages contain a special global network address (0xFF) that means the message is intended to be read by all nodes. Certain J1939 message (those in PDU 2 format) can only be broadcast if their data length is 8 bytes or less.

In the situration you're describing, the controller would probably broadcast the tachometer information using some PGN that is defined by the J1939 standards as containing tachometer data. Even though it is broadcast to all nodes, those nodes not interested in that PGN can ignore it. The tachometer gauge would be programmed by the manufacturer to detect that PGN and extract the tachometer reading from the message's data.