[Coco] FDD 26pin to 34pin

Chester A Patterson vchester at setec-cr.com
Mon Aug 7 16:42:04 EDT 2006

I see. Way back when, I had two 5.25" 360k and one 3.5" 720k running in
different configs and it was great (but as I think about it, never with
the 3.5" as FDD0)

I see what you mean about current FDD PC drive ID scheme, I bought two
brand new ALPS 1.44, however I was able to get at the solder pads and
change one FDD to drive 0. I was thinking of putting in some pins and a
jumper but it's not worth the bother.

Returning to my project... While I've got plenty of info on PC FDD
connections, cables, twists and what not, I guess what I REALLY need to
know is what signals are at the FDD end of a typical Coco FDD
controller. In my particular case a Disto Mini controller (and a SCI as


-----Original Message-----
From: Mike Pepe [mailto:lamune at doki-doki.net] 
Sent: Monday, August 07, 2006 9:00 AM
To: CoCoList for Color Computer Enthusiasts
Subject: Re: [Coco] FDD 26pin to 34pin

Chester A Patterson wrote:

I'm not sophisticated digitally/softwarewise but do enjoy soldering and
tinkering very much. I assume the 0/1 drive select lines are two bits,
00=FDD0, 01=FDD1, 10=FDD2 and 11=FDD3. These two 26pin FDDs have the
0123 switch. Some 
connections are fairly obvious however I'm stumped at the 0/1 items...
(Head select, Density select, Side select) Where to they connect on the
controller? STOP (pin14) perhaps a typo and should it be STEP? My plan
is to end up having two 5.25 360k (/D0, /D1) and these two 3.5" 720 (D2,

>>>>> <<<<<

No, they're not. They are separate drive select signals. The original
spec has 4 individual drive select lines, but PCs only supported two, so
the other two "vanished". Many 3.5" drives are physically hard-coded to
drive select 1 due to the "twisted" cable select used on the PC. You'd
have to make your own twisted cable to use a PC floppy as drive 0.

You really can't do what you want due to a glitch in the radio shack
controller design. What the CoCo calls drive select 3, the rest of the
world calls side select. In a nutshell, you can only use 3 double sided
drives. If you want to use 4, you'd have to hack something yourself and
patch it.

I don't think there's much point though. Given the choice I'd move over
to the 720k media exclusively. One 360k drive and two 720k drives would
be sufficient to transfer the old disks and give you the opportunity to
read and write them when the need arises.

> There's been much here in the last week about FDDs, controllers and 
> High Density. I find I greatly enjoy this discussion. Does anyone have

> the schematics for the FD500, FD501 and FD502 controllers?

I should make a web page with the schematics I have.

Incidentally there is some verbage on an Intel floppy controller
datasheet I have that warns about mixing 5.25 and 3.5" drives on the
same cable. This is due to the differences in termination and the bus
driver difference that's used between the two types of drives. That
being said, I've never seen a problem with using both on either a PC or
the CoCo.


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