Vehicle data bus (CAN bus)

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Using the CAN Test Box

If you don't have the CAN Test Box, see "Testing Without the CAN Test Box" below.

Firstly connect the 16-pin plug of the CAN Test Box into the DLC (Diagnostic Link Connector) located on the vehicle as shown in Figure 1. The LEDs on the CAN Test Box will begin to illuminate, notifying you that communication has been established and also to show you which pins are active on the DLC you are connected to. It is important to ensure that the following pins are illuminated as this will indicate that the CAN Test Box is powered up and functioning correctly:

Battery V+: Pin 16

Chassis GND: Pin 4

Signal GND: Pin 5

Using the leads provided with the CAN Test Box, connect the YELLOW lead to Channel A of the scope and connect the YELLOW banana plug to pin 6. Then connect the BLACK 4 mm banana plug to pin 4 to provide the ground for the scope. Connect the RED lead to Channel B of the scope and connect the RED banana plug to pin 14. Then connect the BLACK 4 mm banana plug to pin 5 to provide the ground for the circuit. The connections are shown in Figures 1 and 2.

Pressing the spacebar on the PC will start the scope displaying live data. You may need to switch on the vehicle ignition. The CAN-H and CAN-L waveforms will now appear on the screen, as shown below.

Testing Without the CAN Test Box

Plug one BNC test lead into Channel A of the oscilloscope and another BNC test lead into Channel B. Connect a clip onto each of the black (ground) plugs on the BNC test leads, and attach them both to the vehicle battery negative terminal or a good ground point on the chassis. Attach one of the Back-pinning Probe to each of the coloured plugs on the BNC test leads. Using the vehicle technical manual, identify the CAN-H and CAN-L pins at an accessible point on the CAN network. (Usually available at the multi-way connector at each ECU on the network.) Carefully probe the back of the multi-way connector, using Channel A for CAN-H and Channel B for CAN-L. Pressing the spacebar on the PC will start the capture of live data. The vehicle ignition may need to be switched on. The CAN-H and CAN-L waveforms will now appear on the screen, as shown below.

Example waveform

Waveform notes

In this display, we can verify that data is being continuously exchanged along the CAN bus, and it is possible to check that the peak to peak voltage levels are correct and that a signal is present on both CAN lines. CAN uses differential signalling, so the signal on one line should be a mirror image of the data on the other line. The usual reason for examining the CAN signals is where a CAN fault has been indicated by OBD, or to check the CAN connection to a suspected faulty CAN node (ECU). The vehicle manufacturer's manual should be referred to for precise waveform parameters.

The following CAN data is captured on a much faster timebase and allows the individual state changes to be viewed. This enables the mirror image nature of the signals, and the coincidence of the edges, to be verified.

Typical CAN-H and CAN-L waveforms in detail

Here we can see clearly that the signals are equal and opposite, and that they are of the same amplitude. The edges are clean and coincident with each other. This shows that the CAN bus is enabling communication between the nodes and the CAN controller unit. This test effectively verifies the integrity of the bus at this point in the CAN network, and if a particular ECU (node) is not responding correctly, the fault is likely to be the ECU itself. The rest of the bus should work correctly.

It may be necessary to check the condition of the signals present at the connector of each of the ECUs on the CAN Network, as a final check. The data at each node will always be the same on the same bus. Remember that much of the data on the Network is safety critical, so DO NOT use insulation piercing probes on CAN bus lines!

Technical information

CAN bus is a serial communication system used on many motor vehicles to connect individual systems and sensors, as an alternative to conventional multi-wire looms.

CAN is an acronym for Controller Area Network. It is becoming increasingly common on passenger cars and commercial vehicles. Advantages include significant weight savings, reliability, ease of manufacture, and increased options for On-Board Diagnostics. Disadvantages include increased cost, and the need for some specialised knowledge when servicing and repairing the vehicle.

The heart of a CAN bus is the CAN controller. This is connected to all the components (Nodes) on the network via the CAN-H and CAN-L wires. The signal is differential: each of the CAN lines is referenced to the other line, not to vehicle ground. This has significantly better noise rejection when used in electrically noisy environments like motor vehicles.

Each network node has a unique identifier. Since the ECUs on the bus are effectively in parallel, all the nodes see all of the data, all of the time. A node only responds when it detects its own identifier. For example, when the ABS ECU sends the command to activate the ABS unit, this unit responds accordingly but the rest of the network ignores the command. Individual nodes can be removed from the network without affecting the other nodes.

Since many different vehicle components may share the same bus hardware, it is important that available CAN bus bandwidth is allocated to the most safety-critical systems first. Nodes are usually assigned to one of a number of priority levels. For example, engine controls, brakes and airbags are of the utmost importance from a safety viewpoint, and commands to activate these systems are given highest priority (1) and will be actioned before less critical ones. Audio and navigation devices are often medium (2) priority, and simple activation of lighting may be lowest priority (3). A process known as arbitration decides the priority of any messages. In practice, to the user, all actions appear to be immediate.

Most motor vehicle CAN networks operate at a bus speed of 250 kB/s or 500 kB/s, although systems operating at up to 1 MHz are available. The latest vehicles use up to 3 separate CAN networks, usually of different speeds connected together by gateways. For example, engine management functions may be on a high-speed bus at 500 kB/s and chassis systems run on a 250 kB/s CAN bus. Housekeeping functions such as lights, ICE, satnav and mirrors are on a separate low-speed, single-wire LIN bus. The data on one of the three networks is available to the other two networks through gateways to enable, for example, the transmission to get data from the engine management system and vice versa.

CAN bus is becoming increasingly common on today's vehicles, and will become more common as the technology matures and reduces in cost.

Additional Information - CAN Test Box

The 16 pins of the DLC are available on the CAN Test Box and are numbered as follows:

Pin 1: 485A (Manufacturer's Proprietary Information)
Pin 2: Bus + Line J1850
Pin 3: Future Upgrade
Pin 4: Chassis GND (GROUND)
Pin 5: Signal GND (SIGNAL)
Pin 6: CAN High of SAE J2284
Pin 7: K Line of ISO9141-2 & Keyword 2000485A
Pin 8: Future Upgrade
Pin 9: 485B (Manufacturer's Proprietary Information)
Pin 10: Bus - Line J1850
Pin 11: Clock
Pin 12: Future Upgrade
Pin 13: Future Upgrade
Pin 14: CAN Low of SAE J2284
Pin 15: L Line of ISO9141-2 & Keyword 2000
Pin 16: Battery Voltage V+ (Voltage Supply 4 Amp. Max.)

Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • CAN test box

    CAN Test Box

    £149.00|£249.00|£199.00

  • Back-pinning Probe Set

    £29.00|£48.00|£39.00

  • Flexible Back-pinning Probe

    £4.00|£7.00|£6.00

  • Battery Clip

    £3.00|£5.00|£4.00

  • Large Dolphin/Gator Clips

    £6.00|£10.00|£8.00

  • Premium Test Lead: BNC to 4 mm, 3 m

    £29.00|£48.00|£39.00

  • premium test leads

    Premium Test Leads: Set of four leads 3 m (TA125 - TA128)

    £109.00|£179.00|£139.00

Share your experience

If you have any suggestions to improve this guided test please do so using the 'add comment' button.

Add comment

Your email address will not be published. Required fields are marked *

Guided test: CAN L & H