CAN physical measurements explained
Our measuring devices with measurement on the physical layer (CANobserver, CANtouch, CAN-Bus Tester 2) have the ability to measure and to show the signal levels of every CAN node. From the measurements of quality levels, it is possible to draw conclusions about possible existing problems with the node or with the wiring. They show this physical values for every CAN node:
- general quality level (0 … 100%)
- disturbance-free voltage range (minimal disturbance-free differential voltage
- edge steepness (worst rising and falling edge of the message)
- oscilloscope display with message analysis of the complete message (not in CANobserver)
Beyond that, the CANtouch can measure common mode voltages. Read about this in a separate post.
Measuring the physical bus characteristics
The CAN bus uses a difference signal, i.e. the actual data signal is transferred via two lines inverted to each other (CAN_H and CAN_L). The difference between these two lines generates the signal digitized by each CAN bus transceiver. Any faults occurring there can endanger the correct detection of the bit stream. The CBT2 provides an evaluation of the difference signal in the form of a general quality value, the disturbance-free voltage range and the edge steepness, as well as using the oscilloscope display. All these measurements are determined within a message frame.
Contrary to the quality level providing a general evaluation of the signal quality for the bus, the determination of the disturbance-free voltage range and of the edges, as well as the oscilloscope display are useful aids for targeted troubleshooting.
Disturbance-free voltage range
The disturbance-free voltage range is understood as the disturbance-free range of the differential voltage, which is determined over a certain part of each bit1 of the message frames of the station to be measured. This part is called the evaluation period. Each bit is sampled 64 times. The disturbance-free voltage range is evaluated over 44/64 of the bit width (68% of the evaluation period). At the beginning and the end of each bit, 10/64 each of the bit width are excluded from the determination of the disturbance-free voltage range (see Figure 2). Signal overshoot and settling processes are excluded from the interference voltage measurement if they lie outside the evaluation period. A voltage drop of less than 1/64 of the bit time during the evaluation period cannot be detected reliably anymore and in this case, also has no influence on the determination of the disturbance-free voltage range. (During the arbitration phase (start bit + ID + RTR), as well as during the acknowledge phase, there is no evaluation of the physical properties of the message frame, as several bus stations could be driving the signal during these phases.)
To evaluate the transmission properties of the bus cabling and of the individual CAN bus stations completely, in addition to the disturbance-free voltage range, the edges must also be studied; shallow edges can also prevent the correct decoding of the transmission signal. The CBT2 determines the edge steepness for rising and falling edges separately. To this end, two thresh olds are set at 10% and 90% of the signal level determined (determined differential voltage at 42/64 = 2/3 = 66%, see Figure 2). The time required by the difference signal to change between these voltage threshold values is measured both for falling and rising edges. The rise and fall times are determined with a resolution of 1/64 of the baud rate used and displayed as a value between 0/64 and 64/64. This value always specifies the slowest rising and falling edge of the measured frame. A measured value of 0/64 means that the level change takes place in less than 1/64 of the bit width.
General quality level
The quality level is a generally valid expression of the signal quality on the bus. It represents the most important physical properties of the bus and summarizes these in a single value. The quality level is specified as a percentage value. The value range is 0…100%. The value is determined from the following three components important for the signal quality:
- Edge steepness
The edge steepness is measured as x/64. A steepness of 0/64 represents an ideal edge steepness and is valued at 100%. The worst flank is defined as 32/64 and equates to 0%.
- Disturbance-free voltage range
A disturbance-free voltage range of 1.0 V is defined as 0% – a value of 2.2 V as 100%.
Reflection is the relationship of the disturbance-free voltage range to the peak-to-peak voltage. If the peak-to-peak voltage is identical to the disturbance-free voltage range, this is the ideal case and represents 100%. If the peak-to-peak voltage is twice the value of the disturbance-free voltage range, this is defined as 0%.
All three components contribute equally to the calculation of the quality level.
Oscilloscope display with frame analysis
To evaluate signal transitions and for the measuring of reflections, the CBT2 records the signal waveform of the message frame and presents it in the oscilloscope display of the CBT2 application software. Sampling is performed at 64 times the set baud rate over a total of 160 bits (64 x 160 = 10,240 points). The trigger point for the recording always lies on the first edge of the relevant message frame with an advance of 10 bits. The trigger point can be set in the application software
Hendrik Stephani has developed capable tools for fieldbus diagnosis for 20 years.