[Coco] question about 3.5 inch floppies

[John E. Malmberg]

Stephen Adolph wrote:
> Hi,
> 
> Another dumb question-

Not dumb, but if there is a COCO Hardware FAQ it belongs on it.

> A 3.5 inch drive will work like a 720k drive simply by placing a DD
> (720k) disk into the drive, right?

As long as it is a real 720K disk and not a 1.44M disk formatted as 720K.

> So, to use a 3.5 inch floppy with Nirtos-9, one merely needs to have
> it be plugged in, and make sure to use 720k floppies.   The disk drive
> will operate correctly. (?)

Yes.[1]  It will even work with ROM Disk Basic up to the capacity that 
Disk Basic recognizes, provided you have Disk Basic format the disks.

If you use a 512 sector (MS-DOS default) and Windows hard coded value, 
the ROM disk basic reads and writes the 512 bytes, however the first 256 
bytes goes/comes where the ROM expects, and the second 256 bytes of the 
sector is read/written from the verify buffers.  So it is possible to 
use a PC to read and write files to that a COCO disk can read with the 
right program.

If you have a PC running MS-DOS 6 or earlier, it can read and write 256 
byte sector floppies.  That API was removed from later versions of Windows.

> Interestingly, I have 3 different versions of the same Panasonicn
> JU-257.  Only one of them can be selected to be drive 0 or 1.

Cost saving measure.  All IBM PC clones only use a 3.5 inch floppy wired 
as drive 0, and use the cable twist to make it drive 1.  (It has been a 
while since I looked at it, and I could have that reversed, and I seem 
to remember that on a PC all floppies are configured to be drive 1.)

Apparently someone thought that having the assembler set the drive 
number result in too many cases of them setting it wrong.

Originally PCs had 2 5.25 drives, both configured as the same drive #, 
and the twist was used to change it.  When 3.5 inch drives were added, 
the convention was still followed.

> I think I have read that, in the coco, you want to use hard drive
> selects...no twisted cables.

The twisted cable just swaps the drive select 0 and 1, so depending on 
your configuration, you may or may not need it.

You can actually put 3 floppies on a floppy cable and they should be 
supported by all OS's on the COCO.

What ROM basic uses as the drive select for the 4th floppy is the side 
select line for double sided floppies.

[1]The older drives had termination resistors that either had to be 
added or removed.  The newer drives use auto-termination logic.  For 
energy saving / chip heat saving, you want to make sure that the floppy 
farthest from the controller has termination (either auto or resisters), 
and the others are either auto-terminating or have the resisters removed.

-John
wb8tyw at qsl.net
Personal Opinion Only