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sbRIO-9627 CAN Transceiver Slope Control

Hello,

 

I am attempting to communicate with an sbRIO-9627 via the CAN bus and have run into problems with the physical bus. Despite a clean message being received on the differential bus, the message which appears on the RX pin of the transceiver is completely erroneous. The attached image shows both of these waveforms (blue as differential, yellow as received. Apologies for the low quality.)

 

I believe the problem has something to do with the transceiver itself. The PCA82C251 has a slope control pin (pin 😎 which is being held at Vcc, which the datasheet seems to suggest is a sort of standby mode. It seems that something is preventing the transceiver from operating correctly, but I am unsure whether it is necessary to somehow enable the transceiver in software (I couldn't find anything suggesting this) or whether the board itself is causing problems.

 

Any advice would be appreciated.

 

Graydon Wilson

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Lets discuss your transmission medium.

  1. Do you have the 120 ohm termination at both ends only?
  2. What wire are you using?  Is it low capacitance and 120-ohm impedance wire?
  3. Avoid stacking loops of wire unless they are shielded and tied to ground at only one end.
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Thanks for taking a look at this.

Our physical bus meets typical CAN recommendations. 120Ω termination resistors on each end, stub lengths below 5cm, 22AWG Tefzel twisted pair transmission lines. No other device on the bus seems to be having an issue transmitting or receiving messages (from sources other than the sbRIO). No substantial reflections or noise show up on the oscilloscope when another device is driving the bus.

-G

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