[Coco] 1.2 MEG DRIVES might be ot

[Mike Pepe]

Phill Harvey-Smith wrote:
> Quoting Mike Pepe <lamune at doki-doki.net>:
> 
>>
>>>
>>> People keep saying this, but I've been using switchable 40/80 track 
>>>  drives for about 20 years now and never encountered this problem :)
>>>
>>
>> You've been very lucky, or maybe you're not doing what we think you are.
>>
>> If you bulk-erase the disk first, it'll probably work. If you've got
>> old 40-track data and write new data over it with a double-stepping 80
>> tracker, the half-width new track and the other remaining half of the
>> old track will make a mess that's not readable by another 40 track-only
>> drive.
> 
> Humm thinking about it, it was a switchable drive so effectivly an 80 
> track even in 40 track mode (with respect to the disk head width, which 
> I believe is the issue here).
> 
>> I suppose if you never interchange with real 40 track drives, it will
>> probably work ok.
> 
> Well, ok this is on a PC, but wrote some data to 360K disks on a 1.2M 
> drive at the weekend (transfering stuff to my IBM XT !), and that seemed 
> to work fine, though the disks where initially formatted on the XT. One 
> thing to note here is that the XT in question does not seem to have the 
> original full height IBM drive, but has a half height TEAC drive, ill 
> dig out the model no when I'm at home.
> 
> It is possible of course that later model drives have the same heads as 
> 80 track drives, and so would probably have no problems exchanging data 
> with them, this may be worth checking out, though alas I have no really 
> old drives :)
> 
> Cheers,
> 
> Phill.
> 

Also, Phill, it could be that the PC writes the same data twice to both 
"subtracks" of an 80 track disk. Though that seems like there would be 
slight skew effects and it might not work right.

Though now you bring this up in the PC context, I vaguely recall making 
360K disks on a PC with a 1.2M drive and being able to read them, the 
only explanation I can think of is double-write.

In the CoCo world, the simple double-step probably won't work, and I 
don't think the WD17xx controller is capable of writing the same track 
data to the two subtracks. Though someone may know a sneaky trick or two.