[Coco] [OT] TRS-80 Model I on DE1

[Mark McDougall]

On 20/02/2014 10:02 PM, Mark McDougall wrote:

Just want to make something super clear here.

In my implementations of the TRS-80 Model I and the Coco 1/2, the SD card 
emulates the (IDE) hard drive, NOT floppy drives.

Both the TRS-80 (via Peter Bartlett's IDE interface) and Coco (via Cloud9 
SuperIDE controller) communicate directly with IDE interface registers. This 
makes it very easy (trivial) to connect the FPGA implementation up to a 
Compact Flash (CF) operating in TrueIDE mode (the default mode) via the 
Opencores IDE controller component. And this is exactly what I did initially 
on alternate hardware with a CF slot.

Then I thought of developing a front-end module for the SD card that 
emulates an IDE *device*. That way, nothing in the existing architecture 
changes; I just need to add an extra FPGA component between the OCIDE module 
and the physical pins of the SD card. And it could be retro-fitted easily to 
any other FPGA implementation that talks to IDE! There are a few clumsy 
details but - viola!

My colleague wrote the SD card interface, but never finished it, only 
getting the reads working. He assures me that writes aren't handled much 
differently, and wouldn't take much work to add. But I've never bitten the 
bullet to learn how his interface works. FTR we both write VHDL quite 
differently.

So, this is all very different from emulating floppy disks on SD card. It 
would actually be relatively easy to emulate a *single* floppy disk, or 
perhaps even 4 floppy disks, on a single SD card, especially given the fact 
that I do have a WD179x implementation already. However, it would NOT be a 
simple matter of copying DSK images onto a FAT-formatted SD card; rather 
you'd need special software to write RAW disk images. And all that, with 4 
images at most, isn't exactly practical. But it would give you the one 
advantage that the SDC has over the other interfaces, namely truly 100% FDC 
compatibility.

And I re-iterate; to emulate floppies on the DE1 you need another processor 
reading the FAT filesystem and injecting sectors from a DSK file into the 
'raw' disk stream, much like MESS does for the C64 (using G64 files). And 
(possibly) similar to the SDC itself. At least if you want to support track 
commands too!

Hope this makes things a little clearer on the whole DE1 SD floppy issue!?!

Regards,

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