ijor wrote:Drive select tricks might help. But again, other ways of reading might help as well.
Anything seems worth a go! I'm particularly interested in a solution using standard hardware only, as it makes it easy for anyone to set up. Even the requirement of having 2 floppy drives is a problem, as more motherboards are limiting to A: only (if B: isn't in the BIOS, it usually isn't physically accessible).
I tend to believe that the problem is the index, and not the short sync bytes as Pera suggested (which btw, that's quite easy to test by creating a similar situation in the middle of the track).
The TRS-80 issues matches the ST problem, and shows that disabling the index is a way around it. The index halving cable (http://home.planet.nl/~srahman/trs80hw1.htm
) makes the sector visible every other pass. There are also reports that completely disconnecting it works, though the controller appears to need to see at one index pulse to initialise before it can be completely removed.
But more than likely that the problem is the proximity to the index falling edge, and not because the mark detector is off during the whole index active period. Because otherwise you would have problems with almost every 10 sectors per track disk.
The thought behind the drive select change was to remove the index early, though switching back would just bring it back again. I had hoped to try this tonight but I didn't get around to it. Another one was blipping the motor briefly, which might delay by fractionally enough to make a difference. I'm not sure if that's the same idea behind twaddle in the Linux floppy driver, or a completely different fiddle.
Depending on the drive aligment the culprit header might appear further, closer, or even at the other side of the index pulse.
I do remember seeing headers that wouldn't always show up as the first header when scanning a track (starting from the index pulse), but would show at the end of the track but with a time from the index that was greater than the disk rotation time. They might also fall into the borderline case, where they are only sometimes visible, so a repeated read would eventually get them.