Re: CAN-H and Vbatt Short

The TJA1040 is a high-speed CAN transceiver and this means it will act as I told you in my first mail.

When the bus is idle or the node is sending an "1", the transceiver is expecting voltages on both CAN-Hi and CAN-Lo of 2.5 Volts. As long as the bus is idle and you shorten the CAN-Hi to the battery, probably nothing goes wrong; CAN Lo will follow the CAN-Hi to the battery voltage (maybe you can see it with a scope). However when it starts sending a message (Start of Frame means a "0"), the CAN-Lo will go down to 1.5 Volts and the chip will try to get CAN-Hi to 3.5 Volts, which is not going to happen, because of the shortcut. Maybe this is still not seen as an error, but as soon as the state changes to "1" the CAN-Lo will not be back to the baterry voltage when it is measured by the sending node itself. This means errors on the bus. The error counter goes up to 128 and becomes error passive. Allthough it does not send active error frames now, it still does not succeed in sending correct messages, which means the error counter counts further until 255 and this is the end of the story: it goes Bus-off.

Kees

  ----- Original Message ----- 
  From: PDE-Elect - Software Team 1 
  To: 'CANLIST' 
  Sent: Tuesday, December 18, 2012 10:52 AM
  Subject: Re: [CANLIST] CAN-H and Vbatt Short



  Hi Kees, 

  I am using TJA1040 CAN transceiver, And speed of the bus is 500kbps, terminated by 120ohms. When i do this CAN-H and Vbat Short communication is happening in proper. But some error frames are generating during short. and this error frames leading to "Error Passive" state. 

  Hi John, 

  2 nodes are connected in the bus but my device has only Tx messages. No Rx mesages from partner node. 

  This case is mentioned in the document "ISO 11898-2". 

  But first of all i want know whether this error will happen OR should n't happen. If it happen pls give me some reference.. 

  Regards,

  BoopathiRaja.G 



        "Kees Zagers" <[email protected]> 
        Sent by: [email protected] 
        18 - Dec - 2012 01:22 PM Please respond to
              [email protected] 


       To <[email protected]>  
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              Subject Re: [CANLIST] CAN-H and Vbatt Short 

              

       



  Sorry forgot one thing.

  I presumed you have a CAN high speed transceiver chip. If you have a low speed transceiver chip and further low speed network and did the termination in the right way, the network should work with the shortend CAN Hi to battery.

  Kees

  ----- Original Message ----- 
  From: <[email protected]>
  To: <[email protected]>
  Sent: Tuesday, December 18, 2012 8:40 AM
  Subject: Re: [CANLIST] CAN-H and Vbatt Short


  > Hello BoopathiRaja.G,
  > 
  > The CAN controller can have 3 states:
  > 
  > Error Active (normal, Rx cnt < 128; Tx cnt < 128)
  > Error Passive (Rx cnt > 127 or (Tx cnt > 127 and Tx cnt < 256)
  > Bus Off (Tx cnt > 255)
  > 
  > So your node is not in the Error Passive mode, but in the Bus Off mode. 
  > It will not recover from that, until a hard reset takes place.
  > 
  > It is logical that it comes directly into the "Bus off" state when you 
  > shorten the CAN-Hi to the battery. It will try to send a pattern of 0's 
  > and 1's, but it will read back all 0's. CAN-Hi minus CAN-Lo will allways 
  > be more than 2 Volts in difference, so everytime it does a retry of the 
  > first message an error occurs; 32 retries and the controller goes 
  > bus-off.
  > 
  > Hope this clarifies your situation.
  > 
  > Kees Zagers
  > 
  > PDE-Elect - Software Team 1 schreef op 18.12.2012 07:01:
  > > Hi Techies,
  > >
  > > I am facing one issue in my CAN-physical layer design.
  > >
  > > I am using Fujitsu micro. I have a CAN messages only for Tx from my
  > > micro. No Rx messages for my system.
  > >
  > > When i am shorting CAN-H line and Battery voltage(11V to 14 V) line,
  > > my controller generates the "Error passive" error.
  > >
  > > Why this is happening.? And my design validation team doesn't agree
  > > this point.
  > >
  > > And then Error Passive doen't recover back to Error Active State even
  > > if i removed the Short.
  > >
  > >  Pls help me out Is there any protection circuit vailable to avoid
  > > "Error passive " State. Or If it happens in protocol suggest me some
  > > documents for reference..
  > >
  > > Regards,
  > >
  > >  BoopathiRaja.G
  > 
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