Month: September 2017


Part 2: J1939 CAN Network testing and Operation with a Labscope

Using a Labscope to check the J1939 network:

The network can also be checked with a labscope. The waveform pattern displayed on the scope will be a digital waveform of both the CAN Hi and the CAN Lo data signals. The CAN lo signal will be an inverted image of the CAN Hi signal. To properly check a 2 or 4 channel scope will be needed. For the pictures shown a PICO scope and a Freightliner chassis with an ISB Cummins engine were used. 

Note: Pico Scopes can be used to check many other components on automotive, transit and trucking vehicles. It is a versatile tool in todays complex vehicles.


j1939, can HI can LO, CAN scope

  The picture above shows the typical waveform displayed when checking the network, the two waveforms should be mirror images of each other.


j1939, CAN Hi, CAN Lo, waveform transition

  The voltage on the CAN Hi data line should be 2.5 to 3.5 volts, the CAN Lo data line should be about 2.5 to 1.5 volts. The voltage difference is about 2 Volts peak to peak with about 1 Volt between the ON/OFF transition of each data line.


j1939, CAN, CAN Hi, CAN Lo, waveform

  The transitions between ON/OFF of each data line should be crisp with no oscillation or hashing, as shown above. Causes of fluctuations, oscillation, or hashing can be: missing EOL, bad insulation/shielding, network not properly grounded.


j1939, CAN, network, CAN Hi, CAN Lo, EOL

  The picture above was taken with an EOL resistor missing, as you can see the signal has become somewhat erratic, voltage fluctuations and spikes are noticeable during the data lines ON/OFF transitions.


Note: When using a Pico Scope, whatever system that will be checked needs to base-lined. That is, a recording of a good know pattern to compare to the pattern of the vehicle that has an issue. The Pico Scope has a library of patterns to compare, but the user can create their own.