[Coco] high density floppy controller mods

[Joel Ewy]

Hey Frank,

I thought about things like that back in the day, especially after Chris
Burke's XT HD interface.  Why not just make that into a more general ISA
bus interface?  (Maybe it already is more general than I realize?)  Of
course then I was even less capable of getting all the details together
to actually make it work than I am now.  I have some XT and AT passive
backplane systems.  I always thought it would be cool to make a 6809 or
68000 CPU board for one of those.  Put OS-9 in EPROM and have a built-in
serial port on the CPU board so you could use it with a terminal while
hacking out drivers for all the ISA bus hardware.

But one reason to stick with a FDC chip from a CoCo interface is
compatibility with existing software.  ISTR that the command/register
set in the 1773 & co. is different from those of the NEC765 and its ilk,
used in PCs (and the MM/1).  I'm not sure how different they are
though.  If there is anything like a 1:1 correspondence between the
commands and registers then it might not be too difficult to patch
existing routines.

Another evil thought I had was that one could use a state machine and a
ROM to translate the command and status data in real time, so the CoCo
would think it was talking to a 1773, but its commands would be
converted to those of the 765.  Hey, I've got to have something to do
while standing in the shower!

As for using it in no-halt mode, that would be complicated by the fact
that there's ISA bus interface circuitry integrated in the chip between
the FDC and the CPU.  No-halt controllers for the CoCo, like the Disto
SCII and the Sardis controller have what amounts to a simple DMA
controller that fills a sector buffer on the controller.  That part
would be easier to implement if you could get in between the address
decoder and the FDC.  But on a multi-I/O chip, those are all
integrated.  I suppose one could use an i8237 (or whatever the DMAC from
the PC/XT was) and an external SRAM.  But by the time you've gone to
that much effort you have lost a lot of the simplicity gained by using
the multi-I/O chip in the first place.  (Which of course would be a
problem with my real-time translation scheme above...)

On the other hand even though (as some people have already mentioned in
this thread) the chips themsleves might be currently unavailable as new
parts, I have a box of ISA cards with the surface mount parts already
conveniently soldered on.  If one was going to consider making something
like this available to others who still use CoCos, considering the size
of the market, there shouldn't be too much difficulty in finding I/O
boards in the rusting hulks of old '386/SXes lying around all over the
place.  Design a generic ISA bus interface for the CoCo, and treat the
multi-I/O cards as generic PC hardware -- write drivers for the lowest
common denominator of multi-I/O functionality.  The result would be
something functional and cheap, but ugly.  And there's a place for that.

But at this moment I'm more intrigued by the idea of designing new
hardware that requires only incremental modification of existing
CoCo/OS-9 software, and that physically fits into a CoCo system in an
aesthetically pleasing way -- like specifically in the enclosure for a
Tandy or Disto floppy controller.

JCE

farna at att.net wrote:
> Joel, what puzzles me is why no one has ever attempted to use one of hte PC multi I/O chips to build a no halt controller. The chips with floppy, IDE, parallel, and serial ports built in. I recall the cards for ISA slot machines being rather small. I'm not sure you can get 8 bit MIO chips anymore, but surely a buffer arrangement for a 16 bit chip could be worked out. The only problem I can really see is that it may only work with OS9 and not DECB due to the limited room to modify the ROM code. If only the disk controller worked for DECB that would be an accomplishment though. No-halt operation is only good for OS-9 anyway. I'm sure these chips are available, some of the single board controller/computers use them. There are quite a few 16 bit single boards, not sure if any 8 bit are made anymore. 
>
> --
> Frank Swygert
> Publisher, "American Motors Cars" 
> Magazine (AMC)
> For all AMC enthusiasts
> http://farna.home.att.net/AIM.html
> (free download available!)
>
>  -------------- Original message ----------------------
>   
>> Message: 4
>> Date: Sat, 05 Aug 2006 13:21:01 -0500
>> From: Joel Ewy <jcewy at swbell.net>
>> Subject: Re: [Color Computer] [Coco] high density floppy controller
>> 	mods
>>     
>  
>   
>> Mike Pepe wrote:
>>     
>>> James Diffendaffer wrote:
>>>       
>>>> So is there a summary of these mods somewhere?
>>>>
>>>>         
>>> here:
>>>
>>> http://www.doki-doki.net/~lamune/computers/coco/hd-floppy/
>>>
>>> -Mike
>>>
>>>       
>> Very cool, Mike. 
>>
>> My 3029 has the MB8877_ <-No 'A'.  Anybody have any idea if this is
>> significant, speed-wise?
>> I'm very tempted now to get out my Disto schematics and bash them
>> together with yours to get a high-density controller with a sector
>> buffer for no-halt operation, and a 28-pin EPROM socket or two.  If I
>> did attempt to build such a thing, I would start over on a new PC board,
>> just using the MB8877 chip and the data separator from the original
>> controller.  I've got a homebrew CNC PCB drill press that's just waiting
>> to punch holes in some copper-clad...
>>
>> JCE
>>
>>     
>
>