Re: CAN Message Priority

Hi Jeff,

     I am too lazy to do the actual math to figure out how many 8-byte 
data CAN messages can be sent in 50ms at 125kbaud, and have
no time to look it up, so I'll approximate for easy math.

     *About* 125 bits/ message at *about* 125000 bits per second is 
*about* 1000 messages per second or 1 message per millisecond.

     50 messages per 50 milliseconds...   That's much more than 40, and 
the since higher priority messages see no collision, they
won't be retransmitted for 50 milliseconds.   Your lowest priority 
message will either start in a gap between messages, suppressing the
higher priority nodes from even starting their transmissions, or it will 
have 39 collisions to back off from by the higher priority nodes.

     If jitter/latency isn't a big problem, my guess is that if every 
node on the network "plays nice", it should be fine.

     If I'm wrong, a correction will surely follow (from more helpful 
folks) in this message thread.

Rufus

Rufus V. Smith
Faria Watchdog
[email protected]

On 12/3/2012 1:33 PM, jtolmanserra wrote:
> Hi,
>
> I am new to CAN and am working on a project that has 40 identical nodes and a master. Each node is a motor that has two encoders. The two encoder values, each 32-bits, will be transmitted in PDO #1. PDO #1 is configured in each node to transmit acyclically with an inhibit time of 50ms. Network speed is 125K.
>
> I know that the network speed will have to be bumped up, probably to 250K or even 500K. My understanding is that it is the COB Id that determines the message's priority for getting onto the network. I am currently using default COB Ids (PDO #1 is 180h + Node #). It then makes me think that the lower the node number, the higher the priority, correct?
>
> So one of my concerns is that with 40 nodes all sending their PDO #1s every 50ms, is it possible for the higher nodes to NEVER get to send their PDO? If so, then how can I get around this? Should I use the SYNC method? And isn't that going to slow things down a bit? I'm just trying to understand the limitations of CAN and what the best method is for a system of 40 slave nodes.
>
> Thanks. I appreciate all answers or suggestions!
>
> Jeff
>
>
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