CAN Bus Testing with PicoScope 6
The examples in the Maths Channel Basic Tutorial might look contrived but they can be very useful. Use A * B to determine the instantaneous power in a circuit if one of the channels is a current and the other a voltage: V * I = P. You can even choose Watts as a unit.
CAN Bus Signals
CAN is a digital, differential signal bus. A stream of 1s and 0s is sent along two wires.
Logic 1
When a logic 1 (called the recessive state) is sent, both output drivers in the sending device are switched off. As a result, terminating resistors cause both lines to settle at 2.5V.
Logic 0
When a logic 0 (called the dominant state) is sent, both output drivers switch on. The high-side CAN-H driver pulls its line high (to about 3.75V) and the low-side CAN-L driver pulls its line low (to about 1.5V).
The CAN receiver recovers data by subtracting the voltage on CAN-L from the voltage on CAN-H. That means that when a 1 (recessive) bit is sent, the difference is 0V. When a 0 (dominant) bit is sent the difference is 3.5 – 1.5 = 2V. A subtracting PicoScope Maths Channel can be used to reject CAN bus noise and extract the signal just like a conventional CAN receiver.
Noise Rejection
The question arises – why use two wires to send the same information? The answer is noise immunity. Any external noise (usually created by an electric or magnetic field) can be superimposed on a conductor and can destroy the data. If 5V of noise was added to a single-ended (single wire) transmission, it would swamp the data and errors would result.
| Logic Level | Nominal Voltage | Noise | Actual Voltage | Receiver | Noise | ||
|---|---|---|---|---|---|---|---|
| CAN-H | CAN-L | CAN-H | CAN-L | A - B | A + B | ||
| 1 | 2.5V | 2.5V | 0V | 2.5V | 2.5V | 0V | 5V |
| 0 | 3.5V | 1.5V | 3.5V | 1.5V | 2V | 5V | |
| 1 | 2.5V | 2.5V | +5V | 7.5V | 7.5V | 0V | 15V |
| 0 | 3.5V | 1.5V | 8.5V | 6.5V | 2V | 15V | |
| 1 | 2.5V | 2.5V | -5V | -2.5V | -2.5V | 0V | -5V |
| 0 | 3.5V | 1.5V | -1.5V | -3.5V | 2V | -5V | |
The two CAN wires should be (and usually are) twisted together and therefore any extraneous noise is added to both wires at the same time. The receiver cancels the noise out by subtracting the signals.
Even in the presence of ±5V of noise on each signal wire (which is 10 times the amplitude of the data) the receiver will recover the data without any errors. As a result, all ‘common mode’ noise (superimposed on both lines) is cancelled (provided that the receivers can handle the high common mode voltage).
Isolating any Errors or Noise
As can be seen from the table above, the sum (CAN-H + CAN-L) of perfect CAN bus signals is a constant 5V for both 1s and 0s. Therefore, if you create a maths channel that adds the two signals together, any deviation from 5V is twice the amplitude of any noise or error voltage on the CAN bus. The data is effectively filtered out of this noise maths channel which allows you to track down any noise source more easily.