[Coco] high density floppy controller mods

Sun Aug 6 16:30:30 EDT 2006

Joel

IF y ou intend to redo the disk controller or an ISA buss motherboard t hen 
consider the WD2793/2797 FDC instead of the WD1773. It is software 
compatible and is a single chip 5V solution. Like the 1793 and M8877 it will 
do HD 1.44 Meg floppies. 

Also interfacing the coco expansion buss to a fu ll blown ISA 8 bit buss 
would not be that terribly difficult to do. A CPLD with sufficient IOBs will do 
very nicely. 

james

On 6 Aug 2006 at 14:33, Joel Ewy wrote:

> 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
> >>
> >>     
> >
> >   
> 
> 
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> 
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