David Wilson wrote:
Ed Eastman wrote:Anyone want to make a hack to the disk II to put an index hole reader circuit in to alleviate the track synch capture issue? Maybe add it to the EDD board and modify the program a bit? Again, on a IIgs this has potential to becomes a wee bit more powerful.One of my MPF-III disk drives is actually a standard 5.25" half height drive (direct drive spindle motor) with an adapter card to convert the Apple 20 pin Disk ][ interface to a standard 34 pin one. The drive has track zero sensing (no head banging at power up) and will (I expect) have an index sensor as well. Just need some way to get the index pulses out and into the computer (perhaps use the write-protect line instead?)
The write-protect line cannot be read quickly. For the index hole sensor to be useful, it would have to be separately routed to a pushbutton input, where it could be read in 4 cycles. Even that's a problem, since to be useful it must be polled inside the nibble read routine, and another 4 cycles there is a full bit time of additional uncertainty in a polling loop, or another 4 cycles to accomodate in a digital phase-lock loop--likely problematic. Since the rotational position error builds up linearly with time, it should be sufficient to time-reference to a unique pattern on an adjacent track, which would have a small error because of the short seek time. (This technique also avoids the issue of speed changes dependent on the track the head is positioned on--there is much more drag moment when the head is on outer tracks than there is on inner tracks.) -michael NadaNet networking for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool-and it's seriously underused."