Stepper control strategies (I'll eat my hat!)

[Peter Gargano]

> (Now, let's drop this non-GMECM topic.)

What, you want the last word, no chance! Besides,
there's a lesson here, for everyone (and it's the week-end)...

Peter D. Hipson wrote:

> I didn't say they didn't... <g> But to tell when the head is at track zero
> the drive pulls the head back to the stop and then some. 

FD drives have a STEP and DIRN input, and a TRACK0 status output.
FDDs have no local intelligence. The FDD controller, in the PC,
under control of the FDD driver software, controls the stepper 
motor. If the software wishes to ignore TRACK0 status and continue 
stepping out (TRACK0 is outermost track) then there's nothing to stop
it doing this. Depending on the design of the drive, the step signal 
may produce a STEP-OUT pulse when TRACK0 status is active. This action 
usually produce lots of noise as the head carrier eventually contacts
the limit stop, and rattles on it. I've heard this noise lots of times 
when I was playing with BIOS code many years ago.

However, I have never seen BIOS software try to step further than
track 0. Why would it want to?

> Then it sets the track 0 seense output. 

Incorrect. Track 0 sense is usually an opto-interrupter connected
almost directly to the status output.

> next time your PC powers up, listen to it. 

I don't get any noise when I power up my PC, but then again,
I've disable the BIOS option called "Floppy Seek .." or similar that
causes the FDD to step out some and then back, expecting to see TRACK0 
status after it's stepped back, indicating that an FDD is actually 
attached as expected. When I enable this bit, I get a two tone 
bzzzz-bzzzz (not a GRRRIIIINNNNDDDD) on powerup.

> That noise is *not* a problem, it is just the head resetting itself.

Again, strictly speaking, the noise is the head *being reset*
by the PC's software. In this case the term "problem" is pretty 
subjective. Bottom line...

   I hate to be a pedant, but misinformation is just that.
   Perpetuate a myth and it will eventually come back to
   bite you!

Now, if anyone can prove to me that a "properly functioning"
and mainstream PC's BIOS software tries to step past track 0, 
"just to be sure", I'll publicly eat a hat (I have a few) and 
send everyone a picture of it!  Equally, show me an FDD that
"independently" continues stepping after TRACK0 status is active,
and I'll offer to replace it for a new drive.

regards,
-- 
Peter Gargano

PS. My apologies to anyone offended by this discussion, but
interested enough to read down to here (I rest my case :-)!