[Coco] new high density controller

[Phill Harvey-Smith]

Mike Pepe wrote:
> Phill Harvey-Smith wrote:
[snip]

> Problem with the 2793 is that it's not designed to be switchable
> between the 250k and 500k data rates. It has internal analog
> circuitry and would require some analog ugliness to make it work  as
> a dual mode controller.

Humm acording to the data sheet that's what the /ENMF & 5/8 pins are
for, they allow you to run the chip at 2MHz, and internally divide the
clock by 2.

The passives required are only really a couple of pots, a couple of
resistors and a diode.

> Since the HD rates are really only useful in OS9 anyway- you'd need a
>  driver no matter what. So, I think the 1773/82077 hybrid is the way
> to go.
> 
> - It's 100% digital. No pots or passives to worry about, and no 
> calibration to be done.

Though the calibration of the WD chip isn't that hard, it does require a 
scope :)

> - The 82077's FIFO makes no-halt possible

That however would be a defo advantage :)

> without the dozen or so support chips of the DISTO solution. - 

Though I guess you would need more glue logic, unless you use a CPLD or 
something like that.

> It
> would support low density, high density (even 1.2MB drives), and ED 
> disks as well as QIC/Travan tapes. - You could actually have 4 double
> sided devices if you wanted to. 

Yeah that was a weekness of Tandy's interface, though to be fair that is 
down to the extra logic, all the FDC chips used in the CoCo & Dragon are 
capaple already of supporting 4 DS drives, it's just that in the RS-DOS 
case the external logic only supports 3 :(

>- And, 100% compatible with disk basic.

I'll have to go take a look at the Intel's data sheet, got a pointer ?

Cheers.

Phill.


-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.