Re: EMC compatible computers

[John Sheahan <jrsheahan-at-optushome.com.au>]

On Wed, Jan 15, 2003 at 10:08:49AM -0500, jmkasunich-at-ra.rockwell.com wrote:
> 
> Do you mean a dedicated box near the machine, connected
> to a PC that is running EMC?  Or a dedicated box near the
> machine that _IS_ the PC running EMC?

I probably meant the former (I'm a hardware designer (and ex ham too) 
and the  excess stuff in a PC grates a bit)
but excluding the latter would be tricky on any measure of 
applying  "whats the cheapest way to fully solve the problem"
criteria.     I reserve the right to decide case by case..

> > I think the latter is much simpler.  Just because RTLinux
> lets you surf the web while running the machine doesn't
> mean it's a good thing to do.  I for one intend to have
> a PC dedicated to running the machine, with display and
> keyboard set up to be used while standing in front of
> the machine.  Not a nice setup for CAD or web browsing.
> I'll have another PC for "normal" PC work, at a nice
> comfortable desk.  I will use ethernet or floppynet to
> connect the two machines.

Its not the machine I use for normal stuff. The seating is not good
and its too noisy and dusty. :)  
Its also not the machine serving files and net access. Its not the
firewall.    CPU's are cheap. Monitors and chairs and desks cost.

But I would consider non-real-time on the unix box used for
surfing/mail/work while dedicated hardware (rather than rtlinux) does
the rt stuff.  Either is a valid solution - but I think I'd expect a
much longer useable lifetime from the diskless pure HW solution.  

> 
> > parallel ports can't drive long wires, have lousy
> > edge rates, die from ESD, and very occasionally
> > miss transfers.
> 
> Agreed about long wires.  Again I was assuming PC near
> drivers, and short wires.  Noise issues don't worry me
> so much - my day job involves IGBTs switching 1000A at
> 800V in 100nS, near digital and analog electronics.
> I'm confident I can lay out, ground, and shield my
> system to make it work.  For the average machinist
> with limited electronic experience it would be a much
> more inportant factor.

I could probably do too if I tried. But I'd have to work at it, 
and sometimes it would glitch.  I'd have real trouble achieving this
on a lashup prototype.    



> 1)  Allen Bradley 6180 industrial PC.  Picture at:
>     http://www.ab.com/opinter/eoi/icb/rac6180.html
>     200 MHz Pentium, 128MB, in industrial enclosure.
>     Has 14" flat screen, full alphanumeric membrane keypad,
>     all sealed, you can splash it with coolant and metal

Australia generally has a less endowed surplus market unfortunately..

> 
> Used 200MHz motherboards are free, but if you want to
> microstep, you might want something faster.  I like
> the idea of spending $200 for Jon's board, and being
> able to use a $0 motherboard.  Others might prefer to
> put the $200 into the computer, so they have enough
> speed to generate step pulses in software with no
> external "helper" hardware.

Sure - i'd usually choose the HW version too.
I've been doing other things for a while, not familiar with Jon's
design, and he has every right to own his implementation details.

But I would suggest that the hardware description language code (RTL) 
behind the implementation is the thing you want to maintain.  If (when)  the 
FPGA goes away - just resynthesize for the new one.  
Its the hardware version of  source code - just recompile for the
new platform.    Thats what gives  hardware long-term maintainable for me.



> One perspective - there are two kinds of maintainable
> hardware in the long term.
> 
> A)  Simple boards built with commodity parts, with
>     complete documentation.  

Whats a commodity part now?  transistors, inductors, resistors are
fine.   IC's come and go.    FPGA's come and go like any IC.  The RTL 
has worked for 15 years and loosk to have that long left.

(he says with a 120 year old screw cutting lathe in the next room -
which unfortunately reached EOL a while back....)


> C)  Mass market parts that adhere to a standard interface
>     and can be replaced with work-alike units later,
>     even if the original unit is no longer made.
>     IDE disk drives, for example.  This is chancy though,
>     since even "standard" interfaces change and go
>     obsolete.  Try finding a MFM disk drive today...

Hmmm. These are decade-long solutions only. Thats not long compared to
my lathe. Not that I have a much batter solution.

john