RE: [CAD_CAM_EDM_DRO] Re: parallel port finally obsolete?

>For using a feedback related application

>(best example is the THC logic in MACH for plasma cutting) it does not

work.



Not exactly true. The motion control board needs to handle the 'real time'
things itself and in fact can do a MUCH better job than Mach (parallel port)
ever could (due to the limitations of the parallel port model). The
SmoothStepper, et.al, handle things like limit switches, spindle sync, etc,
etc internally.



Jeff Birt

Soigeneris.com



From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Tom
Sent: Wednesday, February 22, 2012 10:16 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: parallel port finally obsolete?







--- In CAD_CAM_EDM_DRO@yahoogroups.com
<mailto:CAD_CAM_EDM_DRO%40yahoogroups.com> , "polaraligned"
<polaraligned@...> wrote:

>
>

> >
> > Forgive me if this topic has been already thoroughly covered - I did a

search and didn't find the answer I was looking for, but I may have given up
too quickly.

> >
> > Being a Macintosh person, I didn't have older PC's with parallel ports

around, and wasn't comfortable evaluating used machines. Just recently an
unused 2-3 year old Lenovo desktop came into my hands, but of course it has
only USB ports.

> >
> > Has the longstanding need for parallel ports now been abolished? If so,

what hardware is now available to interface between USB and stepper drivers?
Is there a choice, and if so, pros & cons of each?

> >
> > TIA
> >
> > Steve
> >
>

You need to be aware of the shortcomings of the USB as a control bus for
CNC. USB is a half-duplex communications method (cannot receive and transmit
at the same time) designed to send non-time sensitive information to/from PC
peripherals It works pretty good in sending things FROM the PC. It polls the
devices on the bus and if they have data the setup time to get it back is
much longer that it takes to actually send it. A device cannot initiate a
call. What that means is that any feedback FROM the system is not fast
enough for real time. Worse than that the delay is variable so you can't
just build in a fixed timing loop.

If all you need to do is get motion from the PC using MACH then the USB to a
translating card works pretty good. For using a feedback related application
(best example is the THC logic in MACH for plasma cutting) it does not work.
Indeed when you load the special driver for MACH it takes to use the USB or
other external pulse solutions you cannot run the parallel port driver or
anything that relies on that for realtime feedback.

While more recent PC's have abandoned the parallel port there is an absolute
Flood of "refurbished" PC's on the market setup and ready to run. A Dell
GX270 /GX280 with a 2.8G processor, 1 G ram, parallel and serial ports
(built in) video and XP Windows loaded can be had for around $100 USD. We
have used upwards of 50 of those models in ready-to-run MACH3 based systems.
Ebay has pages of them. The 620 models work well too and are not much more.
There is a whole REFURBISHER business model happening. Most companies will
refresh the PC's in a 3 to 5 year cycle and that is feeding this supply.

Even a conventional PC with a $14.00 Parallel- serial expansion card is
pretty easy to setup.

Sometimes as engineers (I put myself in that group) we tend to design things
because we CAN not always because we SHOULD.





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