Re: Disk II Phase Question

[Roger Johnstone <rojaws@orcon.net.nz>]

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"