The four status lines that are read/written at base+2 of a parallel port
are open collector. You can arrange for these to be bi-directional without
too much trouble. The IBM spec on these is that there is a  4.7K resistor
in the printer adaptor to pull them to +5. Of course IBM in their
wonderfullness made all but one of these bits inverted.

I have a Compaq Aero where the port does not follow the IBM standard as
above. All other ports that I have tried do, however.

Jim Ham

At 08:32 AM 10/7/97 +0100, you wrote:
>At 09:28 07.10.97 +1000, you wrote:
>>At 01:41 PM 10/6/97 -0400, you wrote:
>>>On Mon, 6 Oct 1997, Gerard Gerritsen wrote:
>>>The i/o lines on the standard parallel port are all unidirectional, so make
>>> sure your signal is connected to an input line.  Also, some of the lines
>>> have inverted logic (low reads high, high reads low).
>>>
>>>Steve
>>
>>Are you sure? I have run FastWire on a 4.77MHz XT. FastWire is one of those
>>fast file transfer programs that uses the parallel port to talk between pcs.
>>It is definitely bidirectional.
>>
>>
>>Regards,
>>
>>Ross McKenzie
>>Melbourne Australia
>>
>>to reply by email remove the "nosp*m." text from my email address
>>
>
>I would expext that it uses som kind of nibble mode.  I think one
>implementation of nibble mode is described in some IEEE1284 document around.
>Although some OLD documentation says that it *IS* possible to read the data
>lines if they are driven by more than 20mA.  Has anyone actually tried this?
>I have done some simple tests, and it semes to work, but I don't think that
>the drivers likes to this kind of drive fight.  The port will probably blow
>up sooner or later...
>
>Havard
>
>
Jim Ham, Porcine Associates
(415)326-2669 fax(415)326-1071
"http://www.porcine.com"