Nearly all of the electronic systems in today's automobiles are controlled and monitored via serial bus links. The differential CAN bus is the primary serial bus used for drive-train and body control. This protocol was developed by Bosch nearly 30 years, and it is still considered the "workhorse" control bus for all of today's vehicles.
The primary measurement tool used today to test and debug the physical layer of the CAN differential serial bus in automotive systems is an oscilloscope. The electrical environment in automobiles is naturally harsh with lots of noise and often unexpected transients. The core competence of an oscilloscope is that it can capture and show you details of those infrequent automotive transients and noise that could be producing CAN bus errors.This document first explains what a .dbc file is. This document then shows an example of an oscilloscope triggering on and decoding a CAN bus (hexadecimal format only) without a .dbc file, and then shows an example of an oscilloscope triggering on and decoding a CAN bus symbolically after importing an industry-standard .dbc file.
DBC databases describe the properties of the CAN network, the ECUs connected to the bus and the CAN messages and signals. You can think of the .dbc file as a translation file. All automotive vendors have created .dbc files for their vehicles' CAN buses.Figure 1 shows a portion of a .dbc file created by Keysight that defines a CAN signal often used for training. Although this simple .dbc file was created using a text editor, the most common tool used today to create more complex .dbc files, which is typical for automotive CAN buses, is Vector's CANdb++ software tool.
To assist in synchronizing on and identifying specific CAN frames (packets of data), most of today's mid-range and high performance oscilloscopes have the ability to trigger on and decode the CAN bus in a hexadecimal and/or binary format as shown in Figure 2.
In this example, the oscilloscope had been set up to trigger on frame ID 0x201(HEX). In addition to decoding the frame ID, the oscilloscope also decoded the 8-byte data field as"0B A8 00 00 27 10 00 00." But what does this mean?
With the new CAN-dbc symbolic decoding and triggering capability in Keysight's 4000 X-Series oscilloscopes, engineers now have the ability to trigger and view messages and signals symbolically by importing an industry-standard .dbc file that defines their specific networks.
Once a .dbc has been created and imported into an oscilloscope with CAN-dbc symbolic triggering and decode capability, the user has the option of either triggering on and decoding the bus in a traditional hexadecimal format, in a symbolic format, or both. This makes testing and debugging the CAN bus quicker and more intuitive.
Figure 3 shows the same oscilloscope now triggering on and decoding the CAN bus symbolically. Instead of attempting to manually interpret cryptic hexadecimal codes, the oscilloscope symbolically decodes it in "human language." In this example, the oscilloscope is decoding a message named "Brake_Torque." In addition, the oscilloscope translates raw bits into signed variables with units, as well as encoded states. For instance, Total_Torque = 131.064 ft-lbs, as opposed to 0B A8.
An oscilloscope is a critical measurement tool used by electrical engineers in the automotive industry to test and debug the physical layer of automotive electronics, including the differential CAN bus. An oscilloscope with the ability to trigger on and decode the CAN serial bus in a symbolic format (via the importation of industry-standard .dbc files) makes testing and debugging the CAN bus more intuitive and ultimately speeds-up the debugging process. Keysight's InfiniiVision 4000 X-Series oscilloscope provides this advanced measurement capability with the DSOX4AUTO license option.
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