CAN phy­si­cal mea­su­rements explained

Our mea­su­ring devices with mea­su­rement on phy­si­cal lay­er (CANo­ber­ser­ver, 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 sepe­ra­te 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 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 any more 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.)

Edge steep­ness

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 as 100%. The worst flank is defi­ned as 32/​64 and equa­tes to 0%.

edge_steepness_formel_en

  • 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%.

distfree_voltrange_formel_en

  • Reflec­tion

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%.

reflection_formel_en

All three com­pon­ents con­tri­bu­te equal­ly to the cal­cu­la­ti­on of the qua­li­ty level.

quality_level_formel_en

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

Hen­drik Stephani

Fiel­dbus dia­gno­sis chief developer

Hen­drik Ste­pha­ni has deve­lo­ped capa­ble tools for fiel­dbus dia­gno­sis for 20 years.

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