CAN-bus must have 120 Ohm resistor termination on BOTH ends of the bus to prevent "reflection noise" ruining the signal (and communications).
This means that DVM measures 60 Ohm
- with module connected to the actual CAN-bus, but power off
- (2 pieces of 120 Ohm resistors in parallel, effectively).
This procedure must always be followed:
In general just measure across CAN+/- with DVM (ignition off)
- if 120ohm, add 120ohm termination (so it will be 60 Ohm). Just a 120 Ohm resistor between CAN+ and CAN-
- if 40 Ohm, there is an extra 120 Ohm termination somewhere (3 in total, there should be only 2)
- 60 Ohm: good, exactly 2 terminators are there. Hopefully at the end of the bus (long CAN-bus will not work if terminators are not at the very end, both ends). Shorter CAN-buses are less sensitve.
If termination is OK (60 Ohm), watch aim2can moduls LED;
- if 1Hz blink all is well
- if 3Hz blink there is some problem (eg. termination BAD, or no acknowledge)
Before ordering
- system prepared for module (eg. with factory ECU disconnected, if it will not be present):
- with ignition off
- If you measure 120 Ohm between CAN+ and CAN-, please
- write "please ship module with 120 Ohm termination" (solderblob closed)
- this way 120 Ohm external resistor (or closing the solderblob in the module, which is easy) should NOT be needed (see above procedure) to get the 2 terminations (and the 60 Ohm desired effective result).
If you measure 60 Ohm between CAN+ and CAN-, please
- write "please ship module with infinite Ohm termination" (solderblob open)
Without explicite information, it makes sense to ship the module with solderblob open (infinite Ohm instead of 120 Ohm termination) by default because it is always possible to install 120 Ohm resistor externally. (or closing the solderblob in the module is also easy).
Remember that these are often customized systems, and there might be other devices on the CAN-bus that may or may NOT have termination. It is always preferred to measure (needed for verification anyway), than to guess.
"To measure is to know" - Isaac Newton