[Coco] MPI and NitrOS-9

[Gene Heskett]

On Thursday 20 October 2016 22:14:12 Bill wrote:

> So, if I were able to build a ribbon cable to connect all four drives,
> I could connect it to one controller? I might try that tomorrow. I
> have the connectors and PLENTY of ribbon cable, but what about the
> twists on the cable? Am I building a cable basically identical to the
> two existing, but with four connectors?

Bill; There are 4 drive select wires in the cable, but one is commonly 
used for side select, so that you can connect only 3 physical drives at 
any one time without useing another controller. Two would allow 6 drives 
for instance. IIRC that would need some tricky stuff not usually played 
with in the descriptors to isolate the two controllers shareing common 
access addresses in the $FF40-$FF48 range.

This means also that there are no missing teeth in any of the connectors, 
nor is a cable twist required.  What is required is that you go thru the 
drives and by solder bridges, flea clips or dip switches, each drive has 
its own unique address from 0 to 2. But the 3.5" drives have only 2 
possible addresses, and as shipped they are all set to address 1, and 
the twisted cable makes the drive on the end of the cable into drive 0 
by swapping those 2 select wires.  Works fine on a doggoned pc, but not 
for os9.

So common practice here at the coyote.den is to have a pair of 3.5" 
drives, with one having its addressing solder blobs reset to make it 
drive 0. The next, is already drive 1, and a 5.25" of your drive choice 
at the drive 2 address. Since this drive can be a 35 track ss, a 40 
track ds, or an 80 track ds, Robert and I worked out a patch for the 
nitros9 rb1773.dr floppy driver such that if a 40 track disk is inserted 
in the 80 track drive, that drive will be double stepped so the heads 
stay in the center of the wider 40 track pattern for reads, and if an 
attempt is made to write to that disk, it will be found to be write 
protected since the narrower 80 track heads will not properly overwrite 
the wider track.  So without the write protection, the disk, which may 
be the only one you have with that data on it, will refuse the write, 
which if allowed, normally destroys the disk and it must be reformatted 
to use it again.

Much of this is better explained by a detailed read of the superdesc.asm 
file in the nitros9 build directory. Because bits in the descriptor are 
hard to find spares for adding these new functions, 1 or more bits may 
control the meaning of 3 or 4 other bits, so read, and re-read until the 
interactions become clear.

dmode, which can adjust a descriptor in memory on the fly, or a 
descriptor on the disk to build the next boot disk, will be your best 
friend while learning all this.
 
> -----Original Message-----
> From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Barry
> Nelson Sent: Thursday, October 20, 2016 3:01 PM
> To: coco at maltedmedia.com
> Subject: Re: [Coco] MPI and NitrOS-9
>
> You don’t need to “merge” them. All you need is a cable that allows
> you to connect both sets of drives to one controller, and set the
> drive ids to different numbers. You could even leave the ids set as
> they are if someone can make a custom cable for you. There are a few
> companies that make custom cables.
>
> You also could use both controllers but I think you may need to patch
> the NitrOS9 disk driver. If it works at all like the original CC3Disk
> OS9 driver, it stores a hex $33 in $FF7F before accessing the
> controller. You would need to patch that byte to change the slot used
> for the floppy controller.


Cheers, Gene Heskett
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