CAN bus wiring test

The wiring test sequence rai­ses ques­ti­ons from time to time. Here we explain why the sequence is as it is. The­re are also a few tips.

For the mea­su­re­ment, you must ope­ra­te the tes­ter with 24V.

The sequence is as follows:

Pre­pa­ra­ti­on for step 1:

• switch off the sys­tem (the bus to be mea­su­red must be voltage-free)
• The nodes must be dis­con­nec­ted from the bus (the input capa­ci­ties could lead to errors).
• You must remo­ve the ter­mi­na­ti­ons (they are mea­su­red later).
• Attach the tes­ter at the begin­ning of the system.

Mea­su­re­ment 1: short cir­cuit test

The resis­tors of the wiring are mea­su­red. They should be infi­ni­te high becau­se no wire is sup­po­sed to be con­nec­ted to ano­ther in any way. For ins­tance, the­re will be an error if you for­got to dis­con­nect one of the ter­mi­na­ti­on resistors.
Short cir­cuits and trans­fer resis­tance bet­ween wires some­ti­mes occur in bad­ly moun­ted plugs.

Pre­pa­ra­ti­on for step 2:

• Con­nect a short cir­cuit plug to the end of the bus. (In a short cir­cuit plug, all pins are con­nec­ted to each other.)

Mea­su­re­ment 2: line-break test

The short­ing plug con­nects all wires of the bus at the end of the cable. The loop impe­dance and the cable length are deter­mi­ned. Values can dif­fer, then you should look for breaks and bad con­nec­tions. If you did not dis­con­nect some nodes, the deter­mi­ned length could vary. To mea­su­re short cables is not easy. The cal­cu­la­ted length is more exact at lon­ger cables.

Pre­pa­ra­ti­on for step 3:

• dis­con­nect the short­ing plug
• con­nect the ter­mi­na­ti­on resis­tor at the end of the bus

Mea­su­re­ment 3: ter­mi­na­ti­on resis­tor at the end of the bus

Con­side­ring the for­mer mea­su­red loop resis­tance, the ter­mi­na­ti­on resis­tor at the end is measured.

Pre­pa­ra­ti­on for step 4:

• con­nect the ter­mi­na­ti­on resis­tor at the begin­ning of the bus

Mea­su­re­ment 4: ter­mi­na­ti­on resis­tor at the begin­ning of the bus

In the last step, the ter­mi­na­ti­on resis­tor at the begin­ning of the bus is mea­su­red by con­side­ring the for­mer mea­su­re­ments of loop resis­tance and the ter­mi­na­ti­on resis­tor at the end of the bus.

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Hints

• Under some cir­cum­s­tances, the nodes can stay con­nec­ted to the bus. The cal­cu­la­ti­on of the bus length will be influen­ced. It is pos­si­ble that you get errors, for ins­tance, when you also mea­su­re power sup­p­ly wires and have a low impe­dance power sup­p­ly connected.
• You might smooth away faults when you move many con­nec­tors. If you assu­me that you have loo­se con­nec­tions, it can be bet­ter to mea­su­re with con­nec­ted nodes first.
• Start­ing mea­su­re­ment 1 wit­hout doing the pre­pa­ra­ti­ons will cal­cu­la­te a resis­tor of about 60 Ohm bet­ween CAN_​H and CAN_​L. This is, of cour­se, rated as an error becau­se, in mea­su­re­ment 1, very high resis­tors are expec­ted. But to quick­ly check the 2 ter­mi­na­ti­on resis­tors, this pro­ce­du­re is good – even if the device rate this as an error.
  It will not be so easy at long trunk lines with hig­her loop resis­tance. Here it would be best if you com­pared it to for­mer measurements.
• The CAN-Bus Tes­ter 2 could be dama­ged if you igno­re the war­nings of the soft­ware and start the mea­su­re­ment pro­cess with a vol­ta­ge appli­ed. The CAN­touch refu­ses the mea­su­re­ment as long as the­re is a vol­ta­ge on the sup­p­ly lines.