Network - CAN - voltage

The purpose of this test is to verify that data is being exchanged along the CAN bus network, to check that the peak-to-peak voltage levels are correct and that a signal is present on both CAN lines and mirrored.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control unit.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Locate the vehicle 16 pin diagnostic link connector.
  2. Connect the CAN test box to the diagnostic link connector.
  3. Connect PicoScope channels A and B
  4. Turn the ignition to ON, engine OFF.
  5. Check the link and test box are active, with coloured LED illuminated for-
    PIN 16 Batt V+
    PIN 4 Chassis GND
    PIN 5 Signal GND.
  6. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Select GO or press the space bar to see live data.
  8. With your live waveform on screen select STOP or press the space bar to stop your capture.
  9. Turn the ignition to OFF.
  10. Use the WAVEFORM BUFFER and ZOOM tools to examine your 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!

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, CAN bus H or CAN bus L.

Further guidance

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.)

GT126-3

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

    £245.00

  • Shrouded to Unshrouded Adaptor

    £2.25

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Guided test: Network - CAN - voltage