AnsweredAssumed Answered

Problem solved.

Question asked by wry.dragon on Jan 16, 2014
Nothing weird in the ISR.
I used a standard example,
 just added a few Pin toggles to follow with LEDs.
Don't see it entering the ISR during DEBUG Mode at all though.
Got it - found the problem !
One of the mcp2551 chips (or both) is not working.

So I use the Diode circuit below, and got rid of the failed mcp2551 chips.
It enters into the ISR properly repeatedly on schedule now !
Data is not always perfect - but can solve that at my ease (via protocols).

Learned Notes:
1) Connecting CAN Tx to Rx on same Chip - will Not work.  It will just continuously Resend without Receiving in the ISR.  If it answered its own message - how could any other Node/Chip answer it then ? 

2) Connecting TxRx to another chip RxTx with common Gnd does not work.

3) Can use Simpler DIODE circuit instead of a CAN chip which works somewhat ok.  Link (2) micros together via a wire connected to the CAN RX lines and Diodes connecting the TX (as a drain) to the same line.  And, using a common Gnd wire. 

  Micro1:                                                 Micro2:
CAN RX <-------+-------------+-------> CAN RX
                          |                   |
CAN TX >--|<--+                  +-->|--< CAN TX
Gnd o-----------------------------------------o Gnd

This gives a lot simpler setup for Debugging.