[Coco] Thought

[Mark McDougall]

Eric J. Rothfus wrote:

> BTW, Mark, if you get serious about doing this, I'd be glad
> to help.  While doing the SVD I've learned a ton about the
> old floppy formats and would be able to contribute a lot
> of software to the effort.  My recommendation after doing the
> opposite with the SVD, is to generate the bit image on the PC,
> and let the simple circuit shift it out.

Well I'd like to do it, it's more a matter of getting the time. :( I'm 
currently juggling several arcade/micro/emu projects and get very frustrated 
at the lack of time I get to devote to each.

My original plan was to emulate the floppy controller for the TRS-80 Model I 
in software running on a soft core inside an FPGA (using SRAM for storage) - 
that would interface to my TRS-80 FPGA implementation. Primarily this would 
be a learning exercise. Next step would be to re-implement the controller in 
VHDL (firmware). Then I could move the storage off-board and I was thinking 
about doing the USB thing at this point.

It's been a *long* time since I looked at the TRS-80 M1/3 floppy controller 
hardware but from what little I recall, it wasn't timing critical in the 
sense that the Z80 issues a FDC command and is interrupted when the 
drive/data is ready. If this is true, then I can't see (in the 5 mins of 
analysis I've done in my head) that 'hiccups' in the USB stream would be a 
problem? Maybe I'm wrong, but it'll all come out in the wash.

Having said that, I've really got absolutely *NO* idea how the CoCo drive 
works. Sounds like it's all polled I/O with strict timing loops in the CoCo 
DOS?!? That sounds yukky and as you point out, could present problems and a 
"USB SVD" may well be the only solution.

FWIW my USB prototype board uses an EZ-USB which itself has an embedded 
12MIPS 8051 and (IIRC) 8KB of RAM. It's got plenty of I/O and although I 
didn't put a footprint down, there's an external address/data bus which 
could accommodate an SRAM, or even serial flash I think might be fast enough 
these days?!? But my idea wasn't to replace the SVD but rather offer an 
alternative that required a 'tether' to the PC whilst offering the ability 
to serve hundreds of disk images or even hard-disk images.

Anyway, very interesting and if I didn't have to work for a living, I'd get 
a lot more of these projects fininshed! ;) If it gets anywhere, I'll let you 
know! Thanks Eric!

Regards,


-- 
|              Mark McDougall                | "Electrical Engineers do it
| <http://members.optushome.com.au/msmcdoug> |   with less resistance!"