[EMAIL PROTECTED] wrote:
> I am puzzled by the issues raised of DMA and interrupts as a problem
> in using USB.
I think that was part of the RT ethernet discussion, but I could be wrong :)
> In the WinXP system a common way of using USB connections is to
> handle it as a 'Virtual Comm Port' (VCP). When you plug in your USB
> device it is assigned a Comm port number. After that the data link
> behaves exactly like a full duplex RS232 link.
>
> Also if you do a Google search you will find several venders selling
> USB to Printer Port converters. These enable printers without USB
> capability to be run from laptops without printer ports.
>
> So why cannot EMC use a USB port exactly like it uses the printer port?
Because a printer doesn't care when it receives the bytes to print, as
long as they don't come too fast. EMC2 needs to change the port outputs
with very exact timing to make a motor move. This is not accomplished
by a USB to parallel converter. Although USB is capable of moving way
more data than necessary, it doesn't maintain the timing between port
writes.
> Obviously you need the equivalent of a WinXP Virtual Com Port driver.
> Is that so difficult to produce?
It's not hardware that matters, it's the timing. There are Linux
drivers for the virtual comm ports, and I suspect for the parallel ports
as well.
> The main attraction of the USB connection is that much higher data
> rates are possible.
This is completely useless unless you can control the timing, which you
can not with USB (unless you make custom hardware). USB has a 1
millisecond frame time. That makes for some very slow step outputs
without custom hardware.
- Steve
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