[Coco] Coco 3 high-density floppy controller/IDE interface

[Gene Heskett]

On Friday 04 July 2008, Phill Harvey-Smith wrote:
>Gene Heskett wrote:
>> On Thursday 03 July 2008, David Hazelton wrote:
>> What does it do with the 3rd ds line?  That was not normally the fdc's
>> responsibility, but was selected via separate latching buffers on the
>> controller pcb.  There were times I had considered replacing the 3 DS
>> lines by feeding them to a 3x8 decoder to get 8 drives & then I wouldn't
>> be moving drive cables all the time to switch from the various flavors of
>> 5.25" drives to the 3.5".
>
>Looking at the schematics in Eagle, and the data sheet for the Intel
>FDC. The FDC supports up to 4 drives, but as the interface is wired up
>for connection to standard PC type floppies, the DS2/3, and ME2/3 lines
>are not connected to the drive chain. However it would be possible to
>connect these so that 4 drives could be supported. Connecting the DS0..3
>  direct to the interface and the ME0..3 through some logic to the
>/MOTOR line. OK this would break compatibility with IBM drives, but
>would allow for up to 4 drives, and IBM drives could still be used with
>apropreate cable twists :)
>
>Another solution may be 2 IBM compatible drive connectors that differ
>just by which DS/ME combinations they have routed to them, so you would
>keep the current one, and add another with DS/ME 2..3 routed to it,
>though I'm not sure if this would work due to it effectivly being a Y
>shaped cable with the controler in the middle.

Not recommended due to signal echo's, doubled terms notwithstanding even though 
this is a fairly slow circuit.  Doubled terms would even be a good idea since 
these cables have about a 120 ohm impedance, but the average FDC chip probably 
couldn't drive a proper scsi termination (130 ohms to about a 3 volt supply) 
without letting out the smoke.

>Cheers.

I looked at it in eagle last night too Phill.  And of the 3.5" drives I have, 
most, with a little detective work, can be addressed wherever you want them.  
Sometimes the artwork is labeled, more often not, and the addressing is cut & 
jump, but its usually there.  For that reason the 4 drive setup looks to be 
doable if the ME2-3 and DS2-3 lines are brought out. I don't know as that could 
be done on all even pins like now or not, but on the 3.5" drives they commonly 
only have pin 1 and 33 as grounds, so that opens up the possibility of using an 
odd numbered pin for that at the controller end.  Drives can always be 
physically patched.

Overall, I like the idea, and I even have some old small ide drives around 
too. :)  My drive tower overfloweth. :)

>Phill.



-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
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What I need is a MATURE RELATIONSHIP with a FLOPPY DISK ...