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

From: Barry S (barry_at_nospam.4.me.thx.com)
Date: 07/31/04 

Date: Fri, 30 Jul 2004 20:32:39 -0500

>> which control network would be, ahem, for lack of a better word, 'best'?
>>
>> (i.e. in terms of price/node, support, ease of development, availability
>> of second-sources, availability of developers etc.?)
>>
>> Personally, being somewhat naive and ignorant of the field,
>> I would go for ethernet, for sheer availability of tools and
>> peripherals. But would I be making a gross faux-pas in doing so?
>>
>> is ethernet gaining ground as a general-purpose control network
>> over e.g. CAN etc.?
>
> To throw in a few more cents (of most likely biased information):
>
> I would not only look at the "best" technology, but also at the
> slightly bigger picture:
>
<snip>

In addition to the good advice Olaf gave you, let me throw in another
perspective:

You never mention what your wiring contraints are (ie space and length
of cable runs). Some of the "serial-style" protocols like CANopen (and
others) are great for minimizing cable runs and providing you a much
greater total length of your runs.

Ethernet, on the other hand, has some limitations regarding cable runs.
Mostly, ethernet would be run using a switch (or hub, if they still
exist). This would affect many logistical aspects of your design.
Namely, can you support many 'home-runs' back to the switch from your
devices -- and are they within 200 FT or so from the switch.

Of course, the serial world has its own set of drawbacks concerning
logistics. Mainly, you need to be concerned with induction. Since each
device is daisy-chained, and if they are in a high EMF and high
electrostatic environment, you will need to look at using optoisolators.
Otherwise, be prepared for a world of pain. Nothing like get zapped and
having all your devices go off line while you run around like a madman
with an oscillator / test equipment.

Moreover, you need to be honest regarding your own skill set. You never
mentioned what protocol you would be running over Ethernet, but I assume
it would be TCP/IP. TCP has a wealth of tools available for diagnostics
(ie. ping, traceroute, etc). Many of the proprietary serial-based
protocols have very little with regard of diagnostics. It can be hard to
find why one particular device won't resond. So, you need to
either "know" that protocol or be prepared for some learning
curves.

>From my own perspective, I've used many types and I've always wished
that I had attributes of the "other type" from time to time. Like you
hinted in your question, there is no *best*, there is only *best for
the task at hand*.

Overall, if you choose a serial-type protocol, go with something open.
Olaf had some very good advice regarding finding numerous suppliers,
ease of documentation, etc. There is nothing worse then needing to get a
part shipped *ASAP* because your network is down, but the only supplier
is closed because its Friday afternoon.

-Barry

>
> 1.) Do you want to use off-the-shelf components, or develop your own,
> or mix both?
>
> If you use ONLY off-the-shelf components, I would make sure that
> whatever you pick is supported by at least 2, better 3 big vendors and
> that their components can be exchanged and that they have "reasonable"
> pricing. Which network technology is actually used then becomes less
> relevant.
>
> If you want to develop some or all of the nodes yourself, you should
> pick a technology "as open" as possible. One of the reasons we focused
> on CANopen is, that it is one of the few protocols that is very
> flexible in regards to the functionality you actually implement.
> Functionality you don't need you simply don't implement. This allows
> for minimal implementations (like www.MicroCANopen.com) greatly
> reducing development time and cost, but also helping in keeping
> per-node costs down, as smaller microcontrollers can be chosen.
>
>
> 2.) You never mentioned volume and price requirements
>
> If it is a low volume application and the per-node costs are not
> terribly important, then again the specific technology is less
> important. However, if you have any per-node pricing restraints, you
> need to start looking at how to save money.
>
> Here CAN and CANopen are still at the lower end of the scale. A
> digital CANopen I/O node can still be built using an 8-bit
> microcontroller. Even with all peripheral chips and glue logic needed,
> the costs of the hardware components can be below $5 (volume
> depending).
>
> Olaf
> Tutor at ESAcademy
> www.CANopenBook.com

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