> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Nelson Hochberg
> Sent: Monday, 26 August 2002 8:59 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: CAN or RS-485 ?
>
>
> My thanks to everyone who has given me advice on this question -- really
> good info.
>

<el snipo>

> I don't see any advantage of I2C over CAN w/18F448.
>
> The cost of RS-485 using the 75176 is certainly attractive.  Using one
> master to poll all the others every second to determine a once in 5-15
> minute event has occurred then the master telling each of the slaves the
> event occurred does not make sense to me.  The suggestions from
> Douglas Wood
> and Dwayne Reid of having a node check the net for activity twice before
> transmitting is intriguingly simple.  I especially like Dwayne's idea of
> every node waiting a different length of time.  I would change that to
> adding the event priority (high priority = lower number) to the node id to
> determine the wait time.  That way, a higher event priority would get
> transmitted first.  I would also include nodes acknowledge all
> understandable transmissions using the same wait scheme with only one
> acknowledgement sent.  This way, node 0 would normally send the
> acknowledgement but if node 0 went belly up, node 1 would take over, etc.
> If a node found the network to be busy, it would wait for the
> acknowledgment
> before starting a transmission.  If two nodes tried to transmit
> at the same
> time, the message would not be understandable; they would not get an
> acknowledgement and would resend.  If I can be convinced this would be
> reliable, it would be the way I'd go.  So Douglas and Dwayne, do you have
> much real experience with this wait scheme?  Are messages lost?
> Does anyone
> else have ideas or even better experience with this?

I prefer polling, as it guarantes (as long as a node does not crash on the
netwrok and pull it down or up) that collisions do not occur.

I have worked in the security industy for many years in access controlling
equipment, and they mainly use 2 types of networks :

RS-485 : Large number of nodes, at 9600 baud can travel a fair few KM's
(longest network I have been involved in was 8KM's with 255 slave nodes)

20mA current loop, each node passes the data on to the next, and forms a
ring. Large cable length, limited number of nodes in one loop due to delay
in each node.

Regards,

Kat.

**********************************************
K.A.Q. Electronics.
Electronic and Software Engineering.
Perth, Western Australia.
Ph +61 (0) 419 923 731
**********************************************
>
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