-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Gerhard Fiedler
Sent: Tuesday, May 01, 2007 9:26 AM
To: piclist@mit.edu
Subject: Re: [PIC] CAN Bus

>> Tom Sefranek wrote:

Thank you, Gerhard!

>> I have an application note from Analog devices that states that you can 
>> only have 40 nodes on a bus.

>You probably know this, but just to state it here: CAN is not a hardware
>specification. The actual hardware layer is defined in other specs (ISO) or
>none at all. In the end, it is the specific hardware interface (chip or
>custom circuit) that defines the properties of the wire interface.

>So this AN from AD may be valid for one of their CAN interface chips, but
>doesn't necessarily say much about others.

I think it is nice of you to take the time to point this out,
It is re-assuring.  As I re-read the MCP2551, it CLEARLY states, 112 nodes
can be connected!

>> All nodes work well for the first 900 or so messages. then the lowest and
>> highest ID nodes in each group of 6 stall, while the middle 4 (ID) nodes
>> continue chugging on. The stalled nodes make some attempt to catch up,
>> but eventually the software FIFOs fill up with unsent messages. It make
>> no difference what ID I assign to the failing nodes.

>> But the highest and lowest ID nodes are the most physically distant (6")
>> in the star stub.

>The two statements above seem to contradict each other: in one you say the
>assigned ID makes no difference, in the other you say there's a clear
>relationship between lowest/highest ID and physical distance. 

Ahhh!  Again I fall victim to ASSUMPTIONS.
The address are assigned in the boot software with the PIC reading it's
physical location on the board, and which physical location on the machine.

>I agree that it doesn't really sound like only a wiring problem. Always the
>same nodes that fail? How about changing their location in the ID sequence
>(not anymore lowest/highest), and/or changing their location in the wiring?

I have changed starting IDs, but have not yet hard coded IDs.
I have moved their locations in the wiring, no change with either.

>> I know this is complicated, and if I knew then what I know now.... I
>> would have designed it as all daisy chain. (But I read the spec.
>> allowing nodes to be quite long off the bus relative to the 3 inches I
>> am using in my nodes.)

>Again, don't mistake the CAN spec for the wire interface spec. For the real
>details, you need to look at the interface itself.

Well, I started with the Microchip AN713, AN738, AN930, AN945, and the
MCP2551 spec. sheet.  Then read the BOSCH CAN Specification, other app.
Notes, as well as CAN errors doccumented online.  I can not sort out what I
have learned from which source.

Gerhard

Again, Thanks!

Tom

  *
  |  __O    Thomas C. Sefranek  WA1RHP@ARRL.NET
  |_-\<,_   Amateur Radio Operator: WA1RHP
  (*)/ (*)  Bicycle mobile on 145.41MHz PL74.4
 
ARRL Instructor, Technical Specialist, VE Contact.
http://hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist