[rescue] Parallel ports [was Re: Slightly OT: ?Bad Cap Saga]

Curious George jorge234q at yahoo.com
Thu Aug 21 17:53:59 CDT 2008

--- On Thu, 8/21/08, Geoffrey S. Mendelson <gsm at mendelson.com> wrote:

> On Thu, Aug 21, 2008 at 03:29:17PM -0400, der Mouse wrote:
> >If someone has a real USB parallel-port interface, as opposed to a USB
> >printer interface that speaks parallel-port printer protocol out the
> >printer end, I'd be interested.  (It would still be substantially less
> >convenient than a real parallel port, but it probably would have uses
> >nevertheless.)
> I have no idea if this would be of any help, but both IDE (PATA) and
> SCSI are parallel ports. I'm sure they have some sort of handshaking,
> which might make your work a lot harder.

A traditional IDE (output) port is little more than an octal latch.
The input side little more than a tristate buffer.

However, I think newer IDE interfaces (UDMA, etc.) also have
control logic to offload the transfer from the CPU.  Presumably,
there are control registers by which you can "dumb it down".

The issue here would be making sure a normal driver doesn't try
to attach to the controller...

> Siverlining, a third party Mac SCSI driver package had an option of
> using timiming loops, I have no idea if a PC SCSI port could be used
> that way. 

I think most modern SCSI ports are far more integrated than
> >This goes back to my remark about the difference
> between a parallel
> >port and a printer interface that happens to speak
> parallel-port
> >printer protocol.
> There are lots of development boards with real parallel
> ports instead
> of a printer port, except for cost, why not use one of them?

I think the problem is "but then I can't have the code
running on my PC and *talking* to my PC"
> >I think you'd also have trouble finding a serial port that could run at
> >parallel-port speeds.  My ROM reader can grab 64KB of ROM contents
> >through a parallel port faster than you can send 64KB over a 115.2Kbaud
> >serial line (I think - it's been a while since I tried it, but I think
> >it's only something like two to three seconds).  And that's with a
> >completely clockless flying-wire rat's-nest breadboard circuit.  The
> >serial-line version requires substantially more complexity (at least a
> >UART, implying a clock, and probably some kind of microcontroller).
> If this is what you want to do with it, you don't need a particularly
> fast computer. There are plenty of PIII computers with printer ports on
> them, you could probably get a 10 year supply for the asking on any
> of the free mailing lists (freecycle, Craigslist, etc), assuming you
> had to nurse them along and they would not last much more than a year. 

Exactly.  For years, I kept a Compaq Portable III around for just
this reason.  I didn't care about the monochrome screen, etc.
I just wanted the ISA slots and "genuine" ISA bus timing.

> To be honest the last time I owned a ROM reader/writer it had an 8 bit
> ISA card interface, but aren't there USB ones on the market? 

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