Re: EMC compatible computers

[jmkasunich-at-ra.rockwell.com]

John Sheahan wrote:

> My picture is a small, dedicated  box near the machine.
> If its a standard motherboard - with a parallel port,
> thats ok.

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 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.

> If its a dedicated chunk of hardware - say a fpga,
> electrically its a lot like a parallel port.
> Either way its essentially co-located with the  motor driver.

As above, I'd like to co-locate both the PC and any
"helper" hardware next to the drivers.  That way, none
of the cables will be very long.

> > "other related costs"?
>
> I was including the motor driver and perhaps the machine..

Understood.

> >
> > > And the parallel port (electrical) performance sucks
> > > compared to any ethernet - particularly optical ones.
> >
> > Do you mean isolation?
>
> isolation/noise/esd in general.
> GbE over fibre still works better in a particularly
> noisy environment.
>
> 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 just dislike parallel ports! :)

Everyone is entitled to their opinion...  ;)

> > The real burden is step pulse generation and
> > encoder counting.  That is best handled by
> > dedicated hardware, not software of any kind.
>
> agree, although modern PC's are quick enough to blur
> the difference.

They are getting there.  It still takes quite a PC if you are
using high count encoders or microstepping.  I would prefer
that the fastest PC in my house be on my desk.  I'd like to
use older, slower, cheap PC(s) to run machinery.

> I still prefer designs that hang in there over time..
> PC hardware is just so ephemeral compared to the stuff
> its controlling, particularly now.

You hit the nail right on the head.

My approach (right or wrong) is to freeze the technology at
some point, and try to be able to maintain that point, rather
than always going for the latest and greatest.

Maintainence is the trick - if the motherboard I use today
breaks 4 years from now, I won't be able to find a replacement
that works the same.

Since I am an incurable dumpster diver, my approach is to use
cheap or free stuff and keep spares on the shelf.

For example, my CNC project.  I will use one of the following
two options - both came from dumpsters.

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
    chips and it doesn't care.  Has connectors for external
    keyboard and mouse.  I have a spare power supply,
    motherboard, display, backlight, video card, and
    membrane switch interface card.  The motherboard is
    ATX form factor, but the BIOS may be semi-proprietary.
    Disks, CD-ROM, etc, are standard PC components.
    By the time I add stepper power supply and drivers,
    it will be a big heavy box that has to be mounted
    where I can see the display.  Maybe I'll remote
    mount the stepper stuff and use a 6' parallel port
    cable.

2)  Cubix BC5200-TS "server blade" single board PC.
    http://64.173.211.2/support/techinfo/bc/ts/intro.htm
    200 Mhz Pentium, 64 or 128 MB, SCSI & IDE interfaces,
    floppy, video, LPT & COM, keyboard, ethernet.
    Everything you'd find on a motherboard.  Plugs into
    a 16 bit ISA slot, but the only thing it needs from
    the slot is power.  I have eleven of these.  If I use
    these, one box about the size of a tower case will
    contain stepper and PC power supplies, Gecko drivers,
    Jon Elson's universal stepper board or my homemade
    version, the "blade" PC, and disk drive(s).  Connect
    monitor, keyboard, and motors and away I go.  Box
    mounts anywhere, monitor and keyboard where easily
    seen and reached.

> used motherboards suitable for emc are generally
> in the free category now.
> fpga based proto boards are not 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.

One perspective - there are two kinds of maintainable
hardware in the long term.

A)  Simple boards built with commodity parts, with
    complete documentation.  That's what I would have
    if I build my own version of Jon's board.
    Jon's own version of the board isn't too bad,
    but the long term availability of the FPGA is
    unknown, and I don't think he supplies schematics.
    These boards can be repaired at the component
    level.

B)  Complex mass market boards that are cheap enough
    that you can stock up on spares.  That way when
    (not if) the vendor discontinues the board, you
    still have the spares.  Dumpster boards qualify
    here, as long as you can get multiple identical
    ones.

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...


This has been an interesting discussion, made me think
about things I have ignored.  Thanks for listening to
my wordiness...


John Kasunich