On Tue, 30 Dec 1997 07:27:43 -0800 John Montalbano <jrmont@iquest.net>
writes:

>3) Same as two, but connect the signal to a parallel port pin that can
>generate an IRQ to the PC on each clock cycle.

Be wary of this one.  Not all parallel port hardware processes the
interrupt the same way.  Because of this hardware inconsistency, nearly
all OS software shuns the parallel port interrupt, polling the printer
with a timer interrupt instead in order to print in the background.

Serial ports are somewhat more consistent in implementation, and the baud
rate generator and transmitter can be used as a reasonably accurate
timer.  However, some ports seem to send stop bits that are not exactly
one bit time in length.  Also some use 24 MHz/13 (1.84615 MHz) rather
than 1.84320 MHz as a time base.