Using a 3.5" 720k disk...

...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 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 --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "SEBHC" group. To post to this group, send email to sebhc at googlegroups.com To unsubscribe from this group, send email to sebhc+unsubscribe at googlegroups.com For more options, visit this group at http://groups.google.com/group/sebhc?hl=en -~----------~----~----~----~------~----~------~--~---