[Coco] CoCo floppy disk subsystem

[Gene Heskett]

On Friday 15 January 2021 09:23:14 Rick Ulland wrote:

> On 1/14/21 6:19 PM, Salvador Garcia via Coco wrote:
> >   I have heard about the WD1793's lack of reliability.
> > Would swapping out the WD1793 with an MB8877A help eliminate
> > reliability issues? Thanks, Salvador
>
> If this is the original controller with three adjustment pots inside,
> that's the infamous analog data separator. I believe the 12v is to
> power this bit. Bound to be off by now, and I think you need a test
> disk to get it back on. Has value as a cool part, but maybe not
> practical.
>
>
> -ricku

If that's the case and if I correctly recall the schematic, the current 
drain is quite low, certainly under 5 miliamps, so surely a 7551 cmos 
version of a 551 could drive a couple 1n914's to make around 9.5 volts 
at enough currant to make it work. None of the old coco's ever made more 
than 10 volts (that I've measured) on the so-called 12 volt buss anyway.

That Fujitsu 8877B we've seen mentioned in this thread will work well in 
a hacked controller to let you make true 1.44 meg 3.5" disk, running at 
half a megahertz, or probably even 2.88 meg disks in a 5.25" 80 track 
drive with QD disks in it. But I was never able to figure out how in 
os9, to set that up so it was automatic.  And I lost the impetus to do 
it when I got my first hard drive.

The J.A.Pan co. was great at borrowing the spec sheet for some of our 
chips, like the 6809 became the much smarter 6309, the WD1793 became the 
8877 etc etc, but both are cmos, and were buckets faster versions if the 
clock speed is cranked up. At some point, the head drivers in the drive 
become the limiting factor. The QD disks needed at least 2x the drive 
than the HD disks did. Their coatings were shaving mirrors and so hard 
magneticly that I wasn't able to format a QD disk for 720k that lasted 
long enough to verify the formatting, but a stock 360k worked just fine, 
at 720k+ in a 80 track drive. I often pushed the track count up to 83 or 
84 on those full height Teac drives.

Cheers, Gene Heskett
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