[discussing the use of IRQ 7]



To put it a little differently:

 - IRQ 7 is the de facto standard interrupt assigned to be used by the
   printer adapter to announce its completion of some activity.

 - DOS doesn't monitor IRQ 7; it uses other means to determine when it's
   time to send out another byte to the printer.

 - Most (all?) (hardware) printer adapters have the ability to disable
   the use of IRQ 7, usually by merely breaking the connection between
   the ISA pin and the associated driver.  Other adapters control the
   IRQ line by a tri-state driver, and by programming just leave it
   in the high-impedence mode.

 - Unfortunately, there are a lot of adapter cards which use bistate
   drivers (i.e., either assert high or assert low) for the IRQ lines
   rather than tristate drivers (assert high, assert low, or don't
   assert anything).  The presence of such a card on an IRQ line precludes
   the use of that IRQ by any other adapter unless it is physically 
   disconnected by a jumper.

   (Incidentally, note that there's no requirement that a card hold
   the IRQ line low when no interrupt is desired.  If that were true
   you would have to somehow tie down all unconnected IRQ lines, and
   that certainly isn't a requirement.)

 - Non-DOS operating systems (OS/2, NT (?), various Unices or whatever the
   proper plural of Unix might be) require the use of IRQ 7 for performance
   reasons.  

And the SB16, alas, is one of the cards which uses bistate drivers.