CAN BUS Transceiver Question

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,430
Good afternoon. I am looking at an automotive type board that has two transceivers on it. One is for the drivetrain and the second one is for the body datalink. There was some water intrusion damage on the circuit for the drivetrain link which I repaired and that circuit is working fine. The other transceiver from what I can tell (non-damaged circuit) is not working. When I check signals on Tx and Rx from the micro, the drivetrain circuit has a nice clean signal. The other one has nothing. I removed the transceiver hoping to see a signal of sorts on the Tx and Rx from the micro. Nothing. Am I approaching this wrong? I was thinking possibly a short in the transceiver so eliminating it may allow me to see a signal from micro???
The transceiver is a PCA82C251.
https://www.nxp.com/docs/en/data-sheet/PCA82C251.pdf
 
Last edited:

Papabravo

Joined Feb 24, 2006
18,432
I'm not sure I can be much help. The 82C251 is more robust and fault tolerant than its predecessor, the 82C250. That said, I would have looked on both sides of the transceiver for activity. That is, bus activity should result in data on the RX line so the controller can detect error frames. Similarly, a stuck TX bit in the dominant condition will result in nodes going bus-off. In all my years of working with CAN I've never seen a transceiver failure.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,430
Okay I can add a bit more to the story. So I have a known good unit and I checked it out and had activity on both sides of both transceivers. When I go to the damaged one, I have the one transceiver whose circuit was water damaged, working and showing a good pattern on both sides of the transceiver. On the one that is not putting out and sign of life on Tx or Rx, there is also no activity on the CAN HI and CAN LO pins on the other side either. That being said, there is nothing currently connected to the unit for it to talk to. I have never seen a transceiver go bad, but I am wondering if there is a reason why the micro wouldn't put out a signal to be seen on the Tx line? Can it be "locked up" somehow or is it possible that it is dead? How would one verify either of those?
I guess I could put it all back together and connect it to a truck but not sure if it needs to be programmed first since VIN will be different? Not sure.
 

BobaMosfet

Joined Jul 1, 2009
1,979
Okay I can add a bit more to the story. So I have a known good unit and I checked it out and had activity on both sides of both transceivers. When I go to the damaged one, I have the one transceiver whose circuit was water damaged, working and showing a good pattern on both sides of the transceiver. On the one that is not putting out and sign of life on Tx or Rx, there is also no activity on the CAN HI and CAN LO pins on the other side either. That being said, there is nothing currently connected to the unit for it to talk to. I have never seen a transceiver go bad, but I am wondering if there is a reason why the micro wouldn't put out a signal to be seen on the Tx line? Can it be "locked up" somehow or is it possible that it is dead? How would one verify either of those?
I guess I could put it all back together and connect it to a truck but not sure if it needs to be programmed first since VIN will be different? Not sure.
And your sure the chip that isn't responding _is_ getting power and/or ground?
 
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