Re: Not enough slots!

[dempson@swell.actrix.gen.nz (David Empson)]

In article <2brj9b$o4i@sleepy.cc.utexas.edu>
foegelle@sleepy.cc.utexas.edu (Michael Foegelle) writes:
> In article <SKNKWRKS.93Nov8160455@mayall.cs.unlv.edu>,
> Scott Alfter <sknkwrks@mayall.cs.unlv.edu> wrote:
> >In article <cJwmcc3w165w@ersys.edmonton.ab.ca>
> >herbertf@ersys.edmonton.ab.ca (Herbert Fung) writes:
> >>Hmm... I wonder, how realistic is it to construct a sort of switchbox for
> >>peripheral cards? Would it be safe? It'd certainly be convienient, one of
> >>my GS's has run out of slots and now I'm using a second one.
> >
> > [Scott mentioned a device which allows switching between four cards]
> 
> Of course, someone COULD design a box that was more than just a mechanical
> type of switching device.  It'd probably need a separate power supply to 
> releave the computer's supply, but all the cards could be powered up and 
> operational at the same time the rest of the computer is online, and then 
> there'd just be buffering circuitry to control which card was actually 
> logically 'mapped' into the slot at any given time.

This may work for some types of cards, but you'll run into problems
with any disk drive interface cards, or I/O card which are currently
"active".

ProDOS-8 locates all disk devices at startup time.  Changing a disk
device in a slot would almost certainly cause a crash if ProDOS tried
to access it.  If you added a disk device, ProDOS would not be aware
of it.  You would have to reboot to get the adjustments to work properly.

The IIgs works somewhat like this with its internal ports and slots -
if you change the slot settings, you have to reboot for them to take effect.

I/O cards (serial, parallel, etc.) are probably OK to switch while in
ProDOS-8, provided you aren't in an application which is using the
card you are switching out (especially if it uses interrupts or DMA).
If in doubt, quit out of the application, switch the slots, then start
your new application.


A IIgs running GS/OS is a different matter.  GS/OS provides drivers for
all character and disk devices.  Switching any such device is likely
to cause GS/OS to crash if the old device is accessed.  The new device
would not be recognised by GS/OS programs.

GS/OS provides a facility called the "Slot Arbiter", which is supposed
to be used to select between ports and slots.  Unfortunately, it has
never been fully implemented, and it only tells you if the desired
port/slot is available.  Even if it did support slot switching, it
wouldn't support changing cards in the external slots.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand