Re: 'best' control network? Devicenet vs Lonworks vs CAN vs Fieldbus vs Ethernet ???

From: Paul Keinanen (keinanen_at_sci.fi)
Date: 07/23/04 

Date: Fri, 23 Jul 2004 22:30:33 +0300

On 22 Jul 2004 16:18:59 -0700, perfb@yahoo.com (perfb) wrote:

>no doubt this is a FAQ oft beat to death, but, assuming
>one wanted to control a network of about 10 i/o modules each
>with maybe 100 i/o digital/analog points, at maybe a 100Hz
>maximum rate (e.g. an automatic industrial processing machine of some kind)

Do you have a simple master (multi)slave system or a peer-to-peer
system ? What is the distribution between the 100 digital+analog
points ? What kind of distances are between nodes ?

If there is only a single master polling all the stations, practically
any system would be usable and still get very predictable latencies.

>is ethernet gaining ground as a general-purpose control network
>over e.g. CAN etc.?

CAN based protocols are nice if nodes transmit messages spontaneously,
since the hardware based arbitration solves most of the problems.
However, the CAN frame can only contain 64 usable data bits (8 bytes),
so if there is a lot of analog points, quite a few frames would be
required.

However, with your requirements, less than 10 frames could be sent
from a single module in each cycle at 1 Mbit/s. However, due to the
arbitration method, the maximum distances become quite short,
especially if optoisolators are used. For systems distributed over a
large area 500 kbit/s would be more usable, but this will enable only
3-5 frames/module/cycle.

Paul