[time-nuts] OT - USB to LPT Adapter - Does it exist?

[paul swed]

As mentioned the real answer is no unfortunately. I used to use raw printer
bits for all kinds of stuff. Not anymore.
Regards
Paul
WB8TSL

On Fri, Jan 11, 2013 at 10:43 AM, Jim Lux <jimlux at earthlink.net> wrote:

> 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<http://www-user.tu-chemnitz.de/~heha/bastelecke/Rund%20um%20den%20PC/USB2LPT/index.html.en>
>>
>>
> Exactly..
>
>
>
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