CAN 8461/2 high baudrate

[MF]

Hello,

I also am having great difficulty with the PXI-8461 CAN card on a Real-Time OS. I can do the loop back and get 50k baud rate to work using the Object to Object.vi example provided with LV 7.1. When I go to 125k it does not work unless I have a 120 ohm resistor across pins 2 and 7 (the High and Lo sides of the CAN signal). Seems like a weakness in the CAN card design as this could be implemented with a jumper on the card.

M. Froehlich
Lakewood, CA

[biker2000]

Hi M. Froehlich,

this is not a weakness of the CAN card's design, but this is actually required by the CAN specification for the high-speed physical layer (ISO 11898-2). Take a look at chapter 4 (page 4-5) of the current NI-CAN Hardware and Software Manual for more information on the termination of high-speed CAN networks.
National Instruments could have included a 120 ohms termination resistor on the CAN port but that would greatly reduce the flexibility of the CAN card as that means that the card has to be connected to the end of the CAN bus (it can't be located somewhere in the middle) and could cause some problems with other devices that are already terminated. E.g. if you already have two CAN devices with built-in termination, you wouldn't be able to add the CAN card to that network.

-B2k

[MF]

OK,


I accept that termination should not be on the board. However, I am still having problems with the card. I have put the termination resistor on the cable connecting the 2 ports on my 8461 card. I can get baud rates up to 1M using your Port to Port 2.vi and the Object to Object.vi. Then I connect the port to the NI CAN Demo Box and I cannot see any messages using CAN Receive.vi. Am I doing something wrong?

Mike

[biker2000]

Hi Mike,

does the CAN Demo Box transmit CAN frames at the baudrate that the CAN Receive.vi example uses? By default, both use 125k, but that default value might have been changed.
Further more, you can (manually) disable all transmission of the CAN Demo Box. Navigate to the 'CAN Transmit' option on the CAN Demo Box (using the 'Menu Select' button) and make sure that it is not set to 'Disable All'.
Also make sure that you connect that port of the 8461 card to CAN Demo Box that the CAN Receive.vi example runs on.

Does any of this help?

-B2k

[MF]

Thanks B2k for all your help. I made sure the baud rate on the CAN Demo box and the software matched. Data came through perfectly. Thanks again.

Mike

P.S. One thing that could be improved is the software for the PXI RealTime and the local CAN. If they could do the same thing as far as monitoring data, that would be a savings as far as debug work goes.