Re: capture incoming messages on the parallel port

From: Michael Black (et472_at_FreeNet.Carleton.CA)
Date: 11/08/05 

Date: 8 Nov 2005 00:38:46 GMT

"Robert M. Riches Jr." (spamtrap42@verizon.net) writes:
> On 2005-11-07, Peter T. Breuer <ptb@oboe.it.uc3m.es> wrote:
>> Dan Jacobson <jidanni@jidanni.org> wrote:
>>> I want to trap boot messages as dmesg etc. doesn't get them all.
>>> In lilo.conf I have console=lp0 console=tty, as my laptop does _not_
>>> have a serial port.
>>
>>> OK, the messages would then come out on a printer attached to the
>>> other end of the parallel cable. But I don't have a printer attached
>>> there. I have a computer attached there instead. What should I do on
>>> that (Debian) computer to trap those incoming messages into a file?
>>
>> Woe, that's weird! I don't think you can do that - surely a printer port
>> is not bidirectional, not with the lp driver, anyway. What driver are
>> you using? Have you tried
>>
>> cat < /dev/lp0 >/var/log/mylog &
>>
>>
>> (see remarks above for why it can't work).
>
> I was cautioned many times ago to never argue with Peter
> Breuer, but I guess I'm a glutton for punishment. :-)
>
> Many printer cables are labeled with an IEEE standard number
> and the word "bidirectional".
>
> I am fairly confident I remember seeing the printer make and
> model automagically detected back in the days when I still
> connected my printer(s) to the parallel port on my Alpha
> machine that was running RedHat 6.1-7.2.
>
> It may be that the lp driver can't handle bidirectional data
> transfer, but I wouldn't be surprised if it were possible,
> maybe with some other driver.
>
The issue is twofold, as always. Hardware has to be two way; this
wasn't the case for the parallel port in the early days of the "IBM PC",
but in more recent years, especially as higher integration came into
play, it tends to be standard. Offhand, I don't know whether the
cable is different; that seems a tad off because most cables I've seen
fill out all the pins.

But then there is the software. What purpose would it serve to have
a two way driver for a printer port? I briefly used a serial Macintosh
inkjet with Linux, and the driver was two way, but it needed that because
it needed some info from the printer that came on the serial data line.
But virtually no printer needs to send data to the computer, the handshake
lines take care of that. If there's no need for receiving data, then
why would someone go to the trouble of making the printer port two way?

That's not to say there isn't some driver around that will listen to
the parallel port and handle data coming in. I would expect there to
be something like that, because there are lots of uses for that sort
of thing. But not a printer driver.

  Michael

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