CAN decoder

The CAN decoder I/O decodes analog data to CAN data. The source of the decoder can either be the differential CAN signal (preferred) or the CAN-high signal. The CAN decoder I/O has auto level detection. The decoded fields can be shown in a table sink.

When the CAN data contains J1939 messages, these are shown as well. They can be additionally decoded using the J1939 decoder I/O.

When the CAN data contains messages that are compatible with CANopen, exta CANopen related fields are shown as well.

The following fields are extracted from the CAN data by the decoder:

Protocol	Field name	Purpose
CAN	ID	(Unique) identifier for the data
	RTR	Remote transmission request: if 1, remote data is requested
	IDE	Identifier extension bit: if 1, the ID consists of 29 instead of 11 bits
	R1	Reserved bit, only in extended format
	R0	Reserved bit (it must be set to dominant (0), but accepted as either dominant or recessive)
	DLC	Data length code: number of data bytes (0-8 bytes)
	Data	Transmitted data (length dictated by DLC field)
	CRC	Cyclic redundancy check
	CRC delimiter	Must be recessive (1)
	Ack	Indicates whether the message was acknowledged
	Flags	Indicates errors in the CAN data
J1939	Priority	Message priority, provides 8 levels, 0 is highest, 7 is lowest
	PGN	Parameter Group Number
	Source address	Source address
	Reserved	Reserved for future use, must now be set to 0
	Data page	DP, selector for protocol data unit (PDU), currently at 0, page 1 for future purposes
	PDU format	PF, 0-239 indicates destination address in PS, 240-255 indicates extension to PF
	PDU specific	PS, content interpreted according to information in PDU Format
CANopen	Node ID	Identifies a device on the CANopen bus
	Function code	Identifies the CANopen message type
	Object	CANopen communication object type

To show or hide specific columns from the table, use the Select columns () button in the Table sink.

Double clicking a row in the table will zoom the active graph in to the time frame corresponding to the table row.

An example of decoded CAN data is shown below.

Measurement on a CAN bus in a car, with decoded CAN data.

Figure 1: Measurement on a CAN bus in a car, with decoded CAN data.

Properties and actions

To control the behavior of the CAN decoder I/O, several properties are available. These can be accessed through a popup menu which is shown when the I/O is right clicked in the Object Tree. The properties can also be accessed through its settings window which is shown when the I/O is double clicked in the Object Tree. To open the Object tree, click the Show object tree button.

CAN control

By default, the settings window only shows the most used settings. When Advanced is ticked, the extended window with all settings is shown. See also the program settings.

Level

In order to decode the analog signal into a digital signal, the CAN decoder I/O compares the analog input signal with a mid level: anything above that level is considered "high" and anything below that level is considered "low".

Hysteresis

To minimize the effect of noise on the signal when comparing the signal to the level, a Hysteresis can be used around that level. Anything above "level + hysteresis/2" is considered "high" and anything below "level - hysteresis/2" is considered "low".

Auto detect level and hysteresis

Enabling Auto detect level and hysteresis will let the software determine a suitable mid level and hysteresis, based on the measured signal. Each time new data is available, the suitable level and hysteresis will be determined again. In streaming mode, level and hysteresis are determined once based on the first chunk of data and remain at these values though out the whole measurement.

Auto detect level and hysteresis is default enabled.

Bit rate

For correct detection, the Bit rate property must be set to the correct value corresponding to the bus that is under measurement. It can be set to several common values, but also to a user defined value.

Hint: For correct detection of the CAN messages, the sample frequency of the oscilloscope must be at least three times as high as the bit rate setting of the CAN bus.