[Coco] Re: [Color Computer] Re: Dragon Disk interface

[Gene Heskett]

On Sunday 27 March 2005 22:29, Phill Harvey-Smith wrote:
>Gene Heskett wrote:
>> On Sunday 27 March 2005 17:48, Phill Harvey-Smith wrote:
>>>No probs :), I recently did a bit of work porting NitrOS-9 to the
>>>Dragon, so if Contiki was ported to the CoCo, would prolly have a
>>>crack at that too. The basic difference was the difference between
>>>the way the CoCo and Dragon access their disk controlers.
>>
>> Can you elaborate on that Phill?  I've not seen much ink on the
>> disk interface diffs.
>
>Sure, this may get a little technical :)
>
>Dragon Dos is based around the WD2797, as opposed to the older
>WD1793/1773 in RSDos. It is mapped into memory at FF40-FF43, there
> is also a disk control register at FF48 this is the other way
> around from RS. The disk register is encoded as follows :-
>
>Bit Function
>0,1 Binary encoded drive number 1..4
>2 Motor
>3 Double density enable
>4 Enable precomp
>5 NMI enable/disable (INTRQ from WD).
>6,7 Not used.
>
>Two inturrupts are used for communication, the DRQ signal on the 
> WD, is tied to the CART signal on the Dragon, this generates an
> FIRQ every time the data register needs data (write), or has data
> for the CPU (read), the INTRQ fro the WD is tied to the NMI of the
> CPU, and gated depending on bit 5 of the disk control register.
> Dragon Dos does not use HALT :) so no kludgy flip-flop reseting :)
> Also doing inturrupt generated disk transfers are not tied to being
> in double desnisty mode, as they are on the RS controler. Also the
> WD2797 is fully reponsible for the side select therfore enabling 4
> drives to be used.
>
>The physical disk format either 40 or 80 tracks, single or double
> sided, 18x 256 byte sectors per track. This can be set at format
> time by giving apropreate parameters to DSKINIT, though I am not
> sure if a 40 track disk can be read in an 80 track drive (never
> tried it as my drive is switchable :) ).
>
>Dragon Dos has an onboard 8K rom containing the disk operating
> system commands, such as DIR, LOAD, SAVE etc, these are similar to
> the RSDOS commands though some have different names e.g. DSKI$ on
> RS Dos is SREAD on Dragon Dos. The Rom also contains some non-disk
> related enhancements for Basic such as a beep command and extended
> error trapping that allows errors to be trapped and handled by a
> basic program. There was also an enhanced version called SuperDos,
> which was produced by a third party supplier after Dragon Data went
> bust, it fixed many of the DragonDos bugs, but also introduced some
> new ones. I have produced disassemblies of both.
>
>I also have a reasonably readable copy of the circuit diagram which
> I could possible upload to the list site, though at some point I
> intend to re-draw it in something more modern :) I am also working
> on a controler based around a WD2797, and a Xilinx CPLD, that will
> hopefully be able to emulate either a Dragon or an RS controler, I
> will share if and when I have it working.
>
>Hope that wasn't too technical :)
>
>Phill.

Not at all Phill, thanks.  One thing I didn't see mentioned was the 
data rate the 2797 can run at.  I don't have any data sheets on it 
here.  Can it do 500 kilobaud and maybe even 1 megabaud?  I'm 
thinking in terms of 1.2 meg and even 2.4 or 2.88 meg disks in the 
usual 3.5" drive for those drives that can support the original I've 
Been Moved 2.88 disks.  Probably not many of those left in 
circulation though.

Also, since that chip has probably been EOL'd for a decade now, how 
are supplies?  More recent, and possibly more capable clones even 
should be considered.  There was one in common useage on many of the 
older wintel motherboards back before it all got integrated that was 
quite versatile, but I don't recall its part number now.

I do see, in the present linux floppy driver that has to handle 
everything that comes down the pike, has formats to as high as 3.84 
megabytes on a 3.5" drive.  But I don't see a lot of chip specific 
stuff there, so maybe I'm looking at the wrong driver src file.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
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message by Gene Heskett are:
Copyright 2005 by Maurice Eugene Heskett, all rights reserved.


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