AVR-Chat

On Sep 5, 2009, at 11:15 AM, Chuck Hackett wrote:

> I preface this question with the fact that I have very limited  
> knowledge of
> CAN bus and DeviceNet technologies (I do have a background in data
> communications from both the EE and software engineering  
> perspective).  I
> have only 'skimmed' the description of CAN bus and DeviceNet on  
> Wikipedia.
> I know that there are Atmel devices such as the ATMega16M1 that  
> implement
> much of the CAN network support in hardware (skimmed the ATMega16M1
> datasheet).
>
> I understand that CAN works well for 'local area' sensor networks  
> such as in
> a automobiles, etc. but can I leverage the Atmel CAN support for a  
> network
> that is physically much larger (i.e.: one with significant propagation
> delays)?
>
CAN is also used in much larger environments (chemical plants,  
semiconductor fabs, etc.).

> I am currently implementing a network of (hopefully) low cost  
> network nodes
> using ATMega16s connected via multi-master, half-duplex RS-485.  The  
> network
> is:
>
If you're thinking about using CAN you want to also use a CAN physical  
layer (for robustness).

> 1) Outdoors
> 2) Length: 1,000s of feet using one pair from a CAT-5 cable (other  
> pairs
> used for other control tasks).  I anticipate breaking the net into  
> subnets
> with intelligent (buffered) 'bridge' devices if the physical layer  
> requires
> it.
> 3) Bus/power surge protection provided by transorbs, etc.
> 4) Data rate: As fast as practical to reduce message latency but I  
> don't
> anticipate needing an excessively high data rate.  For the moment  
> let's say
> on the order of 100 kbs.
> 5) Message Traffic: Most messages are broadcast type messages  
> containing
> status info that any node may consume to support its work, others are
> addressed to specific nodes.  Some messages are more time sensitive  
> than
> others but there are also messages that are sent more or less  
> continuously
> as bus bandwidth allows.
>
> Would it be possible/practical to use the Atmel CAN support in this
> environment?
>
Should be.

Message priority in CAN is handled by the message itself: that is, the  
actual message, taken as a number, is its own priority. So you have  
quite a bit of flexibility (though existing CAN schemes like CANopen,  
DeviceNet, etc. have their own meaning attached to various bits within  
the CAN messages).

-- 
Ned Konz
Seattle, WA
360-629-1091
http://bike-nomad.com