>> Now if only IBM had kept all the input pins on the printer ports the same
>> polarity, I'd be walking on Cloud 9. Wake me up when the ride is over!
>
> They wanted certain levels on the port after a hardware reset,
> Probably found that if all the pins were high or all low the
> printer they had would hang or something, or the select in and
> Strobe line polarities did not make sense at the same polarity.
> Might have something to do with multidropping of the printer
> port to two printers or two computers way back when.

Or consider that it may have been IBM doing at all.  I believe
that IBM was bowing to defacto industry standards -- an oddity
at that time.

IBM targeted the PC at the low end including CP/M.  There was an
already existing standard for printer interfaces using 36-pin D
shape Amp connector.  Centronics started it -- that's why it's
called a "Centronics style" connector.

When they designed the PC, they included a printer port -- not a
general purpose parallel port.  They changed to a DB25 connector
so it would fit through the little add-in card slots.  Otherwise,
they followed the existing standard so that people could use all
the existing printers (both owned & in the stores).

                                                Lee Jones

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