how to detect open circuit on CAN H/CAN L

user,

The TCAN1043 has an nFAULT pin that reports open and short circuit, as well as two other CAN bus faults and other faults on the device. If you need CAN bus short reporting, the TCAN1043 is the best choice.

What kind of project is this?

Regards,

Hello Eric,

my project is about a DC/DC converter on a hybrid car.

DCDC has to response to a car crash while the signal interface with CAN  may broken.

i go through the datasheet of TCAN1043,

it can only report faults like,

CANH shorted to GND,
VCC, VSUP or CANL
shorted to GND, VCC,
VSUP

without the situation of open circuit.

if i miss somehing in the datasheet, please point out or do you a other solution for CANH/CANL open circuit dection.

thank  you very much.

chenqing.

Chenqing,

You are correct, nFAULT isn't intended to indicate an open bus condition, but I was thinking the way the other bus conditions are detected it may work that way anyway. The nFAULT detection circuit samples the differential current on the bus for any bus faults, and then reports accordingly based on a threshold. When I posted my response I was thinking this might also indicate a bus open condition, and it still might, but I'm not sure how reliable it would be given it wasn't designed for with that intention.

If the type of open you are referring to is the CAN node being completely removed from the CAN bus, the quick way would be receiving no ACK bit after a transmission. If the CAN node is disconnected from the bus, no other node would receive the CAN frame and so no ACK bit would be sent. The problem with this is that there are several other reasons why an ACK bit wouldn't be received, so this isn't an obvious case of the CAN node being disconnected from the bus.

Would there be termination at this specific node or is this a drop node that would also lose termination when the disconnection from the bus happened?

Regards,

Hello Eric, Thanks very much for the explanations. We used a split termination and fixed on the internal pcb. It will not lose when car crash as protected by the mechanical housing. And it is a platform project, whether the split termination mounted or not depends on customer’s need. Can you tell me what’s the impact of termination on this kind of can bus demolition? Thank you in advance.

Chenqing,

In general, split termination helps with electromagnetic emissions from the CAN bus and immunity from high frequency noise along the CAN bus. Each resistor and capacitor network from CANH and CANL acts as a low-pass filter, and slows down the edges of the CAN frames.

For this case though, where the CAN node gets disconnected from the bus, it won't matter too much if the termination is split or normal. I was asking because if there was no local termination at the node and the node was disconnected from the bus, your CAN bus waveforms would look significantly different. The difference in single-ended voltage seen on the CANH or CANL waveform could be an indicator of disconnection from the bus.

Regards,