CAN bus wiring test

The sequence of the bus wiring test occa­sio­nal­ly rai­ses ques­ti­ons. We want to exp­lain here, why the sequence is how it is. Besi­des that, the­re are some hints.

The sequence is as fol­lows :

Pre­pa­ra­ti­on for step 1 :

• stop and switch off your plant
• dis­con­nect the nodes from the bus
• dis­con­nect or switch off ter­mi­na­ti­on resis­tors
• con­nect the tes­ter to the begin­ning of the bus

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 the sup­plied shor­ten­ing plug at the end of the bus
  (If you don’t have an ori­gi­nal shor­ting plug : You can sim­ply make your own shor­ting plug by pre­pa­ring a stan­dard plug with shor­ting brid­ges from every pin to every pin.)

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 can 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 or switch on 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 or switch on 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 ter­mi­na­ti­on resis­tors at the end of the bus.

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.
• It might hap­pen that you 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 be not so easy at long trunk lines with hig­her loop resis­tan­ce. Here you should com­pa­re to for­mer mea­su­rements.