Re: Disk II Phase Question

["Bryan Parkoff" <nospam@nospam.com>]

Roger,

    Thank you for explaining.  I think that I begin to understand.  Phase 
0-3 are used to tell the motor by moving the head arm from track to track on 
the diskette.  If head arm is positioned at track 10 and I want the head arm 
to move to track 0, I would have to process by turning on/off Phase 3 
through Phase 0.  The noise might begin if I overprocess Phase 3 through 
Phase 0 about 20 times, but it should be 10 times because it is previously 
positioned to track 10.  Is it correct.

    Do each even/odd track have total ($5F00) 24,320 bytes in nibble?  Is it 
safe to write data to all 70 tracks (even/odd tracks) once, but not more 
than one time?  Then read data from all 70 tracks to see if data is still 
valid.  It is very dangerous to read and write all 70 tracks because of 
overwriting if possible, but it is best to write once while read more than 
one time for valid data.
    Did someone experience to find out that odd tracks have overwritten data 
on even tracks?  How can you explain why it happens?  I only understand that 
each track has bit stream up to 24,320 bytes.  How is it possible that half 
track might overwrite full track?  It may look like low nibble of this byte 
is from track 4 and high nibble of this byte is from track 6 before low 
nibble and high nibble are together into one byte in track 5.  (I know -- it 
does not make sense, but just give you an idea.)

Bryan Parkoff

"Roger Johnstone" <rojaws@orcon.net.nz> wrote in message 
20050113184101578+1300@News.Individual.NET">news:20050113184101578+1300@News.Individual.NET...
> In <nDkFd.17158$iC4.1098@newssvr30.news.prodigy.com> Bryan Parkoff wrote:
>>     Disk II Drive controller card has four phases.  Do only one phase
>> cause  the head arm to move from one track to one track?  For example,
>> head arm may  position at 48th track.  Phase 3 is turned on and turned
>> off.  It caused the  head arm to move from 48th track to 47th track.
>> It looks like that it would  be from even track to odd track.     The
>> manual says that two phases are required so it can tell the head arm
>> to move from even track to even track instead of one phase.  Is it
>> correct?     If we want odd track, then we have to execute three
>> phases once before  we execute two phases to tell head arm to position
>> on each odd tracks.     I have read Understanding Apple //e manual and
>> Benath to DOS and ProDOS  manual.  I do understand everything how it
>> works, but I only don't  understand the concept how "phases" work.
>> Can you please explain in detail how "phases" can be used?  Please
>> look  at my example below that it shows phases in each tracks.     Do
>> each even/odd track have total ($5F00) 24,320 bytes in nibble?
>
> Stepper motors can't be set to an absolute position. All you can do is
> make them rotate one step clockwise or anticlockwise. The steps are
> exact though, so as long as you know where it started from you will
> always know where the motor turned to.
>
> In the Disk II floppy drive there isn't a track zero sensor, so to find
> out where the head is the Apple has to step the head back until it's
> done at least 35 tracks (the number of tracks the Disk II has). The
> rattling noise a Disk II makes when it boots is the sound of the head
> hitting the stop at track zero. Once it's done that the Apple knows the
> head is at track zero and can just step forward or backward after that
> to move it to the correct track.
>
> There is a good explanation of how stepper motors work on this page,
> complete with diagrams.
> http://www.imagesco.com/articles/picstepper/02.html
>
> -- 
> Roger Johnstone, Invercargill, New Zealand
> http://vintageware.orcon.net.nz/
> ________________________________________________________________________
>  No Silicon Heaven? Preposterous! Where would all the calculators go?
>
>              Kryten, from the Red Dwarf episode "The Last Day"