CAN-Bus Operation: Notes

The primary function of the CAN-Bus is to exchange data at a high transfer rate between CAN subscribers. This is accomplished using two signal lines referred to as CAN-High and CAN-Low. Both of these signal lines transfer the same data at the same time. Two signal lines are used for redundancy in the event of a signal line failure.

Voltage on the CAN-Bus is divided between the two data lines. for an average of 2.5 volts per line. Voltage is measured from each data line to ground. Each module on CAN contributes to this voltage.

Fig 1: Identifying CAN-Bus View On Multimeter

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Fig 2: Identifying CAN-Bus View On Oscilloscope

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When viewing the CAN-Bus signals on an oscilloscope, CAN-Low with be approximately 2.5 volts. The signal will be "pulled" low during communication. CAN-High will be at 2.5 volts, but the signal will be "pulled" high during communication. However, the fact that 2.5 volts are present does not indicate that the CAN-Bus is fault free, it just means that the voltage level is sufficient to support communication.