Re: EMC compatible computers

[jmkasunich-at-ra.rockwell.com]

John Sheahan wrote:

> On Mon, Jan 13, 2003 at 06:41:02PM -0500, alex wrote:
>
>> Consider a controller which consists of several
>> low-speed mother boards connected together
>> either through the parallel data link or the ethernet.

> remember the parallel port is dying. ethernet will
> be around longer.

Yes, and for PC to PC comms, ethernet is the right choice.

But how does the second motherboard (presumably running
DOS to generate step pulses) send those pulses to the
motors?  If you use the parallel port, then you are still
facing the "obsolete" factor.  If not parallel port, then
what?

> The cost of interfacing to either
> is tiny compared to other related costs.

"other related costs"?  With today's step and direction
parallel port systems, "other related costs" are usually
nothing more than a breakout board - and even that is a
luxury - you could cut one end off of the parallel port
cable and wire it directly to the drives.

> And the parallel port (electrical) performance sucks
> compared to any ethernet - particularly optical ones.

Do you mean isolation?  Any other "performance" measure
is irrelevant for CNC - if you only need 2Mbs, then
100Mbs is not better, just more expensive.

>> All the real time and close to the real
>> time apps are handled by DOS (Free DOS).
>> Front end is  handled by  Linux.

> DOS isn't real time.   perhaos using interrupts is
> but then rtlinux would work just as well.   And is
> more future-proof i think.

Yes.

> Still - it seems to me offloading the hard real time
> to an offboard processor is nice. the interface to it
> is a lot less important.

Any real time stuff that RTLinux or RTAI on a Pentium
can't handle won't be handled by DOS on a 386 or anything
on a low cost microcontroller.  The real burden is step
pulse generation and encoder counting.  That is best
handled by dedicated hardware, not software of any kind.

>> Dos can be serviced by a 386 single board PC.
>> Linux - some kind of a Pentium.
>> Software cost - 0. Hardware - not very high.

> If I have learned anything - in small qty the sw cost
> is higher than the hw cost.  in large qty this reverses.

Yes, except - if you can use software that is already
written without modification, then its cost depends only
on licensing - with the right license, the cost can approach
zero even in low qty.

For hobbyists, design costs (software or hardware) are
often hidden, and if the design is "fun", we may consider
them to be zero.  Acquiring the actual hardware, on the
other hand, almost always requires money.

John Kasunich