[Color Computer] [Coco] high density floppy controller mods

jdaggett at gate.net jdaggett at gate.net
Mon Aug 7 10:03:15 EDT 2006

On 6 Aug 2006 at 20:29, Mike Pepe wrote:

> jdaggett at gate.net wrote:
> > Mike 
> > 
> > Only FD500 controllers made after about 1982 or so. I have one that has 
> > the WD1973 controller chip. 
> > 
> > james
> > 
> They weren't called FD500 until the very late 80's.

Mike it is my understanding that there were two designs flavors of the FD500 controller card 
( long card) The first was a 5V/12V requirement that used the WD1793/WD1691 and 
WD2143 chip set. This was catalog 3022 first appeared in 1981. This shipped with the single 
ful lheight 5.25 inch drive.  The one that the I have could not have been built before week 
47 of 1981. The first controller card needed 12 volts only for the WD2143 IC. There was a 
redesign of the original to make it a 5 Volt only. The WD1691 and WD2143 were dropped 
for probably two reason, one being cost and second 5 volt only operation. The Five volt 
operation was accomplished by replacing two ICs  with discrete logic. The FD501 came out 
with the COCO2 circa 1983/4 time frame. The FD501 controller card I have was on or after 
week 18 of 1985. It  has the WD1773 and is labeled FD501. I also have a FD502 controller 
card which has the WD1773 controller. 

Going to the WD1773, which is software compatible to the 1793, was a major cost savings. 
It allowed a smaller board and a single chip solution. That is the data separator was done on 
chip with digital data separation instead of the external analog solutin. In theory, when 
aligned properly, the external phase lock loop data separator is better. It is more forgiving of 
drive speeds. It can be designed/adjusted  to  have a wider lock range than the internal 
digital solution on the 177x series of chips.  I would imagine that it was at least a $1 or more 
per unit savings. Good for the bottom line. This did sacrifice the ability to go with higher 
densities than 360K/720K floppies. 

> This board is copyrighted 1984, the date codes on the chips are all from 
> early '84 as well.
> The schematic references the 1793, and is copyrighted in 1983. So, it 
> may be pot luck if the controllers have a 1793, 8877 or 8877A in them.

That is the redesign single 5 volt pack. 

> The 26-3129 (later known as FD-500) all have the 1773 controller, which 
> isn't hackable.

I only know of two conrollers that used the 1773 chip, the FD501 and the FD502. Correct 
that the 177x series will not handle 500Kb/sec data stream. 


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