Using a 3.5" 720k disk...

[Chris Elmquist]

Hey!  That's me!  and I'm on this list too :-)

Chris

On Saturday (04/18/2009 at 04:13PM -0700), Gene Buckle wrote:
> ...and tricking the computer into thinking it's a 5.25" hard sectored  
> disk...in an H8..
>
> [This is from the sehbc Heathkit list - I figured there may be some  
> interested in what he's done]
> -------------------
>
>
> On Friday (04/17/2009 at 07:12PM -0700), dwight elvey wrote:
>>
>> Hi Chris
>>  How did you solve the problem of the speed changes
>> while the disk is spinning? Several people have tried
>> to do this so we are all wondering.
>>  Also, have you checked several different drive
>> manufactures. Each has a little different quality
>> of speed control. Most all 3.5s are crystal or resonator
>> controlled so the average speed is usually right on
>> but the speed caqn drift from revolution to revolution
>> around that value.
>
> Hi Dwight...
>
> I will admit that this little project took a lot more effort than I
> initially expected--  and Carroll W can confirm that!!  We were both
> working on the problem from different directions.  He provided a lot of
> info in the beginning which laid the ground work for me.
>
> I use an ATMEL ATTiny2313 microcontroller and make heavy use of one of the
> timer/counter units.  I have the counter running at 8 MHz/64 = 8uS period.
>
> Then, each time MOTOR ON goes true, I waste two revs of the media
> "figuring things out".  The first rev is completely ignored as I assume
> the media is probably not up to speed yet.  The second rev is used to
> calculate the speed of the media.  I count the total number of 8uS ticks
> that occurred over that rev, divide by 200 (one rev should take 200mS)
> and the result is the number of ticks per mS in "floppy time".
>
> I then use this value to stage the pseudo pulses starting at the next
> real index pulse.
>
> The code is as simple then as,
>
>                 PulseAt00();
>                 PulseAt10();
>                 PulseAt10();
>                 PulseAt20();
>                 PulseAt20();
>                 PulseAt20();
>                 PulseAt20();
>                 PulseAt20();
>                 PulseAt20();
>                 PulseAt20();
>                 PulseAt20();
>
> where each of those is a simple delay that counts the number of ticks
> of the timer/counter to equal the required number of milliseconds and
> then it generates an index pulse.  The timing is done in the hardware
> by the timer/counter unit and I am simply clearing and then polling that
> counter to determine when its the right time to move on.
>
> I have tested with three different brands of floppy drives.  Two were
> a success and one was an abysmal failure.  Sony MPF920 work real nice.
> Teac FD-235F also work great and Samsung SFD-321B are useless--  because
> they have made them so cheap that they do not even support 720K media
> any more.  The density sense switch is completely missing from the drive,
> and they are forced to always run in 1.44M (500kbps MFM) mode.  That's a
> show stopper since for the H-17/H-88-1 we need 250Kbps MFM mode which is
> what 720K 3.5" media uses.  I use 1.44M floppies but then cover up the
> high-density hole with a piece of tape, which makes them 720K media then.
>
> Anyway, I did a lot of testing... a lot of scoping-- with my digital
> scope and logic analyzer and I'm pretty confident this is a reliable
> solution.  The success with "TEST17" on HDOS was the final sign-off and
> that is consistent now with either of the good drives and exchanging the
> media between them.
>
> I make no attempt to support multiple drives with one microcontroller--
> because they are so cheap, I just put one per drive and then I don't
> have to manage state tables for more than one, etc.
>
> I just use the RC oscillator as the clock for the micro too-- so there's
> no crystals or osc to wire up.  Since it recomputes the tick counter
> each time the motor turns on, if the processor clock has drifted, that
> gets accounted for in the recalculation--  but my experience is that
> these AVRs are pretty stable across typical room temp conditions anyway.
>
> I think the thing that stumped me the longest was that I was not
> disabling my INDEX output fast enough when the drive was deselected.
> I ended up with a 74LS38 external to the micro to solve this problem--
> which then also gives me lots of drive back to the controller and a true
> open collector output which can be bussed on the floppy cable just like
> any other drive.
>
> Chris
>
> -- 
> Chris Elmquist
>
>
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-- 
Chris Elmquist