[Coco] FDD 26pin to 34pin

[Mike Pepe]

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, /D3)

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.

-Mike