[Coco] Q: Burke & Burke hard disk controller

[Gene Heskett]

On Tuesday 30 September 2008, Steven Hirsch wrote:
>Dumb question (haven't started trying to set things up yet, so forgive any
>naivety):
>
>If the expansion box has a selector to make exactly one slot active, and
>the hard disk interface is plugged into one of them, how exactly does one
>gain access to the floppy disk?  Are the disk interfaces always active
>regardless of the slot selector?
>
>Steve

No.  The drivers for the hardware are responsible for handling that switching, 
and restoring it when finished, usually to the default slot 3 where the 
floppy controller normally lives.  Most however, read it, save it, and put it 
back to where they found it.

Note that due to a brain fart (they were possibly thinking like gamers only), 
they also switch the IRQ paths, meaning a device not currently selected can 
exert an IRQ in vain, causing loss of incoming data in a comm proggy for 
instance.

The std fix for that that I have been promoting for many years is to jumper 
all 4 slots together at their pin 8, and remove 3 of the 4 pullup resistors 
located along the front edge of the MPI's pcb, otherwise the device trying to 
exert the IRQ pulldown may have the abilities of its output device taxed 
excessively.

Once this jumpering is in place, the IRQ will get through, and the os9 IRQ 
scan will find it, call the driver to service it, which will switch it, do 
its thing, and switch it back when the IRQ has been serviced.  End of lost 
data, even at 9600 baud.

-- 
Cheers, Gene
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