AVR-Chat

Hi Graham,

 

Personally, I beg to differ on the distance. CAN has now been recognised for
use in aviation and on large aircraft it is going to be some cable distance
from some nodes. I have also successfully in the past run on an ROV
umbilical of 1500m with albeit a slower bus rate of 10Kbps but it worked
extremely reliably with all nodes surface and subsea getting the messages
with no loss of transmission. We put transorbs on the line to protect from
the potential of 1100V from the electronics being shorted to the bus if
water got in the connectors. We did find some transorbs expanded to due to
shorting failures but after replacing them, the bus still worked.

 

I have also used RS485 over the same umbilical and yes, it does work very
well but as we increased the number of nodes, the speed at which we could
communicate between them dropped and we had issues with this for control.
With the change to CAN bus we had a faster throughput because we no longer
had the issue of one master in control and sending and requesting the data.
For more modern ROV's we have changed to Ethernet based communications
surface to subsea as we now have fibre optic links but we still use CAN bus
for the inter vehicle control because of the ease of which to add nodes
without any new programming of the master to request data from it.

 

The real reason for suggesting to Chuck the use of CAN is the simple nature
to which you can add nodes and no need for a master to know about them. They
simply send their data as and when they want or are programmed to do so and
any node on the network can do what it wants with that data.! Can also
allows all devices to acknowledge each message whereas with RS484 if you
want to send to multiple nodes, you have to send a separate message to each
or send a broadcast message but each slave cannot simply reply to it as they
can't all send on the bus at the same time, but with CAN any node not
getting the message correctly can indicate this failure and the unit will
resend until all accept it or a bus error is detected!

 

Cheers,
Dave.

 

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of ecros_technology
Sent: 19 June 2010 19:15
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] "RS"-485 vs CAN (was ATMega16 I/O port protection
diodes)

 

  

I've been following this thread, which started as a simple question about
the safe current carrying capacity of input protection diodes and morphed
into tales of lightning strikes damaging equipment, but now I have a
question about the turn it is taking.

TIA/EIA-485 was designed from the outset to operate over long distances and
transceivers are generally designed to be robust in the presence of the kind
of interference and induced high voltages that are common in long distance
data transmission. When used with the UART that just about every
microcontroller has and a simple master / slave protocol (especially where
the choice of a master node is "obvious"), the system is simple, reliable
and easy to monitor and "debug".

CAN was designed for communication on a network within a vehicle. Although
it is eaily adaptable to room or even small building distances, there is
nothing about it that makes it suitable for long outdoor runs. Transceivers
expect to be exposed to interference, but not induced high voltages. The
physical layer protocol is complicated and finicky to set up and you either
have to buy or make some kind of sniffer to monitor bus traffic.

So, whence the enthusiasm for CAN on this project?

Graham.



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