[Coco] Status update and your input on the Floppy Disk Controller emulator for the CoCo

[Steve Bjork]

Earlier this year, work start on a Floppy Disk Controller emulator for 
the CoCo. This device would plug into a CoCo and emulate the functions 
of the Western Digital Floppy Disk Controller and Floppy Disk Drive. In 
place of the Floppy media, the device would use a SDHC memory card to 
store the floppy disk data.

Since the device would emulate the WD1793 and the other components on 
the Tandy Floppy Disk controller, it will work with most color computer 
software without any software patches. (Even most copy protected software.)

At the last CoCoFest, I received a lot of helpful input on the device. 
Some of the ideas ranged from adding a Network interface to talk to a 
host computer to a video circuit for 256 colors. But the best suggestion 
was KIS (Keep It Simple) so the device could be designed and build in 
the shorted time possible.

With the fact that others are working on both USB and Network interfaces 
for the CoCo, there is no need for the FDC emulator to anything more 
than the FDC and SDHC memory card. There will be a PC program for 
transferring data to the SDHC card and back. You could even use 
DriveWire or CoCoNet to talk to Host PC and will using the FDC emulator. 
These options should handle any need to talk to a host PC for getting 
data to and from the device.

The current design uses two micro-controller. The first is a small, but 
very fast chip to emulate the WD1793 and Tandy logic of the FDC pak. The 
second micro-controller handles the track format and data information 
stored in the SDHC card. The total cost of both micro-controllers cost 
less than $10 in single unit quantities.

The FDC emulator will have a number of setup switches and LEDs for 
configuring and status information. The first four switches will be the 
disk write protect switches. (Much like the write protect tabs on a 
floppy disk.) An eight way switch is the Boot configuration selector to 
load one of eight different drive/disk configurations with an auto boot 
option. A special “write protect all” switch that will keep the SDHC 
from being wrote to even if the CoCo start running bad code. (Handy for 
writing test code.) The LEDs will display drive motor on and Access for 
of the four drives plus a SDHC write protected status and access lights.

In addition to the FDC emulator mode, the device can run in special 
transfer mode for maxim throughput to the SDHC card. The applications of 
this mode range from OS-9 drivers to John W. Linville’s CoCo Video Player.

With the basic design of the FDC emulator now locked down, the type of 
CoCo Floppy Disk Controllers needs to be addressed. Both Tandy FDC paks 
are for the most identical from the software point of view. But there 
are third party devices with a few changes here and there. The question 
is, are they compatible (on the software) to the Tandy FDC pak. If not, 
what changes would I need to support to make software designed for them 
to work on FDC emulator?

The next item is the drive select lines. The standard Tandy setup uses 4 
drive select lines to select drives 0 to 3. There was also a 
semi-standard of using drive 4 select line as the head select line for 
double sided drives. Are there any other configurations for the Drive 
Select lines than these two?

Thank you for your input,

Steve Bjork