Re: updated

From: Nicolas Welte (
Date: 2001-08-31 11:15:32

Ruud Baltissen wrote:
> Knowing now that you used a former 360 KB drive means that for a 360 KB
> floppy "1 track = 2 steps".

Not really ;-) I used just the bare drive mech, without the stepper driver
control. At the time I did this, I didn't see another possibility, because I
had no idea how to convert the phase ouput of the the 1541 to the step/dir
input of the drive. And the drive mech itself does have an 80 step motor, but
with a wide r/w head that will not allow the writing of 80 tracks on a disk,
just like a real 1541. It's a TEAC FD55BR, I think, while the HD ones are
TEAC FD55GFR. The conversion from 40step to 80step must be made on that small
PCB with the Shugart connector.

> Hmmm, a weird idea just popped up. I only hope I'm not inventing the wheel
> twice. You stated you used a 360 KB FDD. Would it be possible to attach a HD
> FDD to your 1541 board in the same way? In this way one could use all 80
> steps to save data on a floppy.

This should be perfectly possible, I also already thought about this.

> Remark 1: I don't know if the HD FDD uses 1 or 2 steps/track. 2 s/t would be
> better as that means less SW to change.

It's 1 step/track, just like a DD drive. This means, that a PC BIOS must know
that it needs to issue 2steps/track if it accesses 360k disks in a HD drive.

> Remark 2: is there an adjustment of the current or whatever needed?

It turned out that the stepper in my drive runs better (i.e. much quieter)
when it's only powered with 5V instead of 12V. I didn't do anything about the
r/w head currents, it just works without any changes in my case.

> A thought-avalange:
> - And what about skipping the analogue part of the 1541 and connecting the
> TTL-signals to the according shugartbus-pin?
> - And to hook the diskcontroller/6522 directly to the shugartbus? (Remark:
> requires SW change)

I don't see the difference between these two. Anyway, I thought about this a
while ago, and there's no problem with all Shugart signals except the stepper
motor control. While I could come up with a circuit that converts the phase
output to the step/dir output, I will lose the direct relation between the
phase and the r/w head position. Since there are four motor phases, and each
one corresponds to a fixed position, you can determine with a single STA
command if the r/w head is e.g. on track 1, 1.5, 2 or 2.5. This may end up in
an unstable position at first, but this is used by the 1541's format routine
to position the head: First the stepper is set to track 1 and then the head
bump is executed. After the bump sequence, the stepper register is still set
to track 1. So, the direct phase/position relationship combined with a
mechanical stop can be used to position the r/w head. Since the step/dir
scheme doesn't have this relationship, it needs to make use of an optical
track 0 sensor, which is not supported by the 1541. I thought of a circuit
that would make use of the sensor and would make the 1541 format routine work
well again, but it would not support custom software that uses this
positioning scheme in the middle of a disk. I think the 1541 also does such a
write in it's reset sequence, this is also hard to implement in hardware.

So for a completely software compatible drive, you need to interface the
stepper motor directly. I think there are nice stepper driver ICs that have a
phase input and outputs for bipolar motors (with four wires only, often used
on drives), and these motors are a hell to steer with discrete components
(transistor bridges and stuff).

With little changes in the drive firmware, it would be easy to interface to a
Shugart drive directly, but it would break compatibility with many advanced
copy protection schemes, and probably copy programs as well.

Maybe I'm trying to make such an interface for my tower project: I'm trying
to put a 128DCR into an old AT big tower, but it's not going to be easy, the
128DCR board is so large! The AT board in there was a full size one, i.e. it
had board space on both sides of the keyboard connector. I think I'll get it
in there, since I already got a smaller power supply and I cut away the
lowest 5,25" drive cage.

> I even dare to dream a little bit further: what about the same trick(s) but
> using a 3.5" DD- or even HD-drive? But that makes less sence as IMHO it may
> be is easier to build a 1581 yourself.

And don't forget that we don't want to have yet another new disk format that
is not supported :-)


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