On Thu, 17 Sep 2009, David Braley wrote:
> Date: Thu, 17 Sep 2009 15:42:41 -0600
> From: David Braley <davbra...@comcast.net>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <emc-users@lists.sourceforge.net>
> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
> Subject: Re: [Emc-users] Questions about the parallel port
>
>
> Peter C. Wallace wrote:
>> On Thu, 17 Sep 2009, Eric H. Johnson wrote:
>>
>>
>>> Date: Thu, 17 Sep 2009 15:43:48 -0400
>>> From: Eric H. Johnson <ejohn...@camalytics.com>
>>> Reply-To: "Enhanced Machine Controller (EMC)"
>>> <emc-users@lists.sourceforge.net>
>>> To: "'Enhanced Machine Controller (EMC)'" <emc-users@lists.sourceforge.net>
>>> Subject: Re: [Emc-users] Questions about the parallel port
>>>
>>> David,
>>>
>>> If you are referring to FPGA type PCI bus boards like the Mesa products, you
>>> are not comparing apples to apples when discussing speed. In the case of the
>>> parallel port, the base thread which handles the I/O is run on the CPU. In
>>> the case of an FPGA board, the base thread is effectively run on the FPGA
>>> board. It handles counting of individual pulses from the encoder or
>>> generating the pwm or step outputs. The servo thread then just comes around
>>> periodically to read the current count, or set a velocity value for the pwms
>>> or step generation.
>>>
>>> For example, the Mesa 7i43 communicates over the parallel port, but as an
>>> FPGA device, it can handle much higher speed counters (encoders), pwms and
>>> step generators than the parallel port can by itself.
>>>
>>> HTH,
>>> Eric
>>>
>>>
>>
>>
>> This is also true of the Pico systems parallel port interfaced products. With
>> smart I/O, the parallel port update rate (from 1 KHz to perhaps 4 KHz) is
>> fine
>> for servo or step generation when all the high speed logic is done locally in
>> the external FPGA or processor.
>>
>> Where the parallel port device becomes a limiting factor, even with a smart
>> external device, is when very high sample rates are required (say 5 KHz
>> and >), or lots of axis or lots of fast realtime I/O is required. Then a PCI
>> or some other faster interface is needed. Unless you have a lot of I/O or
>> linear motors, the PCI interfaced cards are not a requirment.
>>
>>
>> Peter Wallace
>>
>
> Well, it just goes to show how naive I am about all of this stuff. I
> only barely understand what the two of you just said. I think what you
> are suggesting is that some of the work needed to control an axis is
> somehow off-loaded to the smarter FPGA device, be it a faster parallel
> port, or PCI bus I/O device. This of course, effectively freeing up the
> CPU on the computer. I'm just guessing here.
Yep thats it.
>
> I was under the impression that the PC running EMC did most of the heavy
> lifting. Are there any good books out there that a reasonably
> intelligent lay person could read to come up to speed on this stuff? I
> just feel that if I don't understand even the basics, I'm not going to
> be able to make informed decisions.
EMC can do everything in software with simple I/O like a parallel port, it is
just limited in speed. With just a parallel port, speed depends on finding a
CPU with good latency. Smart hardware relaxes the requirement for really good
CPU latency, and allows count rates, step rates, and PWM rates/resolutions
that are not possible with EMC and a bare parallel port. For a step motor
driven mill, a parallel port (on a good PC with low latency) may well be good
enough.
>
> This is how I imagine things in my simple world, and why I thought the
> parallel port was not up to the task. I'll just use encoder feed back
> for my example:
Peter Wallace
Mesa Electronics
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