On 1/11/13 7:00 AM, Nathaniel Bezanson wrote:
J. L. Trantham wrote:
Is there a way to connect a parallel port to a computer via USB?
Not a device that shows up as 'USB Print Support' but, instead,
shows up in Device Manager as an LPT port? I have been able to do
it via PCMCIA to Parallel Port adapters but I have never found a
USB device that would do this.
Nope. Look at how the original PC LPT port works -- it's basically a
buffer chip or two, connected to some address decoders, sitting at a
particular spot (0x378) on the CPU's I/O bus. There's simply no way
to abstract that -- it'd be like asking for USB RAM or a USB BIOS
chip.
Actually, though, with modern fast computers, it *is* possible to
abstract it (although tricky and difficult), because the printer port is
SLOW.
You set up that memory area as protected, so an access causes a trap.
The kernel fields the trap and does the needed stuff to control your
fancy LPT port emulator hardware via USB or Ethernet and send/receive
the bits.
After all, that printer port was designed/specified to talk to devices
at no more often than 1 microsecond (that is, you could change the state
of the Strobe line), and practically speaking, with that 4.77MHz ball
o'fire, the strobe pulse was typically a bit longer.
All those LapLink type cables that did high speed transfers between
computers using parallel printer ports back to back ran at transfer
rates around 200 kilotransfers/second, sending 4 bits at a crack each
way, and that's about as fast as you could bit bang.
Ugly? sure
Pain in the rear to implement in software? yep
Requires a very special hardware interface? Almost certainly.
Software written to bitbang the port will have to be rewritten to use
some other form of I/O. For the typical cases of bitbang interfacing,
the FT245R is a very capable little chip, and can be dropped in place
of the parallel port, to talk to legacy hardware. It just needs new
software to take those raw IN and OUT instructions and fire them over
an abstraction layer, which will pass them through the USB stack and
out to the device.
That is, trap the I/O instructions in userspace and use a kernel driver
to emulate it.
There is an exception -- If you're running legacy software under a
modern OS that prevents raw hardware I/O anyway, it's possible to
hook those IN and OUT instructions, and write a generic driver that
passes the traffic over USB. It's slow, unstable, and basically a
miracle if it works. But it's worth a try:
http://www-user.tu-chemnitz.de/~heha/bastelecke/Rund%20um%20den%20PC/USB2LPT/index.html.en
Exactly..
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