>> Incidentally, Microchip is supposed to be coming out with
>> a PIC with a CAN interface.

>Hmm. May have enough poke to run CANbus, but I guess it would be stretched to
>run the application protocols built upon CANbus.
>I don't bother getting excited about chips before see a sample.
>I've seen too many delivery dates redshift into the distance.

> Also, while CAN is a good networking system,
> there are some environments where field busses are NOT a good idea.

>Such as?


You probably already know about this, but ...

Electrically noisy environments can induce enough noise in a network cable which blocks communication.  I heard about people running DeviceNET (Allen Bradley's Fieldbus based on CAN) in the same conduit as high voltage AC.  Duh.  Another system had problems when a VFD motor was turned on.  The problem turned out to be in grounding, but for a while there was a safety problem.

Another limitation is bandwidth.  Good analysis can take care of this, but a lot of plant maintenance people just don't have what it takes to understand it.  (The salesmen tell the manager it will save money, manager buys it, maintenance installs it.  Nobody seriously looks at it in the overall system.)

So what happens if the noise problem doesn't show up for a while or comes from a system added later?  

The advantage of field busses is that connections are simplified.  The disadvantage is that all your eggs are in one basket.  Electrical codes require hardwired E-Stops and Master Control Relays.  You can't hang their output off a fieldbus.


> I'm still not sure if I like the idea of a network controlling the brakes in a car.

You may be too late! CAN originally meant Car Area Network, developed to
reduce the kilometres of wiring in a typical car.
After so much design effort, it was adopted for use in other areas and
renamed as the more generic Control Area Network.
It may well make cars safer and cheaper, as there are less wires
to make a mistakenly connect.

Yeah, I know.  I guess another part of the safety issue is the initial wiring in the system.  Of course, if you're relying on drug addicts to wire them for you, you gotta make it simple. ;-)

> PC as a controller... Think how often your PC crashes,
> then decide if your factory can deal with that kind of down time.

Yep. Dead right!

Sure wouldn't want rely on Windows 90 Something.