Measurement Computing DBK70 - Manual (Page 67)

DBK70 User’s Manual 928494 Fundamentals of Obtaining Vehicle Data 6-1
Fundamentals of Obtaining Vehicle Data 6
General Operation …… 6-1
Obtaining Vehicle Data for J1850, ISO 9141, and ISO 14230-4 …… 6-2
Example Setup from SAE J2190 …… 6-6
Using Diagnostic Data Packet …… 6-8
Obtaining Vehicle Data for CAN, Including J1939 …… 6-12
Data Frame …… 6-12
Creating a Request Message …… 6-16
J1939 Considerations …… 6-17
OBD CAN Considerations …… 6-18
Supported Standards …… 6-18
General Operation
The basic operating premise of the DBK70 is that data about a vehicle, whose value is desired to be
monitored and/or available for recording in the form of a physical signal, is available one way or another
through a data bus in the vehicle. The desired data may be available in a message that is always
re-occurring on the data bus and/or is available if requested, generally as a response to a diagnostic request
message. Since model year 1988, as more and more vehicle data becomes available through the data buses
that are being used in more and more vehicles, this data can be monitored and/or recorded by accessing it
from the data bus through a DBK70. The DBK70 can also provide access to data that is created in
electronic modules as they go about doing their job, data that is only available from inside the module and
not available at all through any add-on sensor.
The data available through a data bus is found in messages that are transmitted and received on the data
bus. Two basic types of messages can be found on a vehicle data bus, operational messages and diagnostic
messages. Operational messages are messages that are transmitted and received by electronic modules
attached to the bus in order for the vehicle to operate properly. These messages normally appear on the data
bus on an ongoing basis at a rate that is adequate to support the data availability needs of modules that need
the data in order to do their job successfully. Operational messages include only the data that is absolutely
necessary for the electronic modules to do their jobs. Diagnostic messages are messages that are used to
obtain data that is available through the data bus but is not available or not adequately available in an
operational message. Diagnostic messages usually involve a request and response message pair.
A diagnostic tool external to the vehicle obtains a connection to the vehicle’s data bus through a diagnostic
connector. The diagnostic tool sends a diagnostic request message to the data bus and receives one or more
diagnostic response messages from the data bus. Vehicle electronic modules that have a connection to the
data bus receive diagnostic request messages, and analyze the request messages to see if the module should
respond to it. If an electronic module is responsible for responding to the request message, it does so and
typically generates a response diagnostic message. Typically, a very large number of data values can be
obtained from a vehicle through diagnostic messages and by comparison, relatively few data values are
available through operational messages.
While many data values may be available on most or all vehicles, particularly new models or future
models, the actual data available will generally vary from model to model, even within the same
manufacturer or product line. Because one model has a data value doesn’t mean another will have it.