[vox-tech] more newbie questions

[Jeff Newmiller]

On Fri, 19 Apr 2002, Joey Karalius wrote:

[...]

> A followup question:  Since the 25-pin SP connection is the same as the 
> PP connection,

No.  The physical connectors are superficially similar, but the electrical
connections they make are quite different.

> can I hook my PP ZIP drive to the SP, and my SP modem to 
> the PP?

No.

>  When is a serial port not a parallel port, and vice versa? 

Always.

> I've got the ZIP drive-to-printer hookup on my parallel port, and want 
> to use my scanner as well, but I understand that daisy-chaining these 
> things slows down performance.  If I expand my port selection with 
> another card,  do I need to get a PP card for PP devices, and SP for SP 
> devices?

Yes.

> (I mean, if the shoe fits...) ;)  I just want to understand a 
> little more 'bout the hardware.

About twenty of the pins on a 25 pin serial connector are virtually never
used.  The few that remain have higher voltages and they toggle on and off
in different patterns than parallel connections.  The obvious wastage was
why no-one complained when IBM came out with a 9-pin serial port on the AT
computer.

On a parallel port, the original design had only unidirectional data lines
intended to send data to printers and a few feedback lines to indicate
paper out and so forth. (Why they chose to use a parallel port for
printers that were at the time phenomenally slow beats me.)  Some creative
hacker(s) came up with tricks for using the feedback signals to turn the
data around and transfer data between two computers.  Later, additional
"modes" of operation were added to the parallel port to allow
"bidirectional" data on the data lines... which the "ppa"
scsi-over-parallel-to-zip driver can take advantage of if it is available.

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