>Paul Koning <paulkon...@comcast.net> wrote: >Does the tachometer have to be accurate, or does it just have to indicate > "spinning fast enough" to satisfy the spin-up logic?
It's not just spin up - the firmware monitors the disk speed all the time it's running. But AFAIK the tachometer has nothing to do with how the bits are recorded on the media. It's just there to be sure the disk is actually rotating. >If not, you could just use a 555 to generate a pulse train ... Maybe, probably, but I'm not sure why you'd want to. That's way more complicated than just fixing the tachometer sensor, and having the firmware shut down the motor in the event the belt breaks or jams, or if the motor brake (yes, it has a brake!) freezes, etc is an advantage. It's a fairly big motor and a fairly (by PC standards at least) heavy disk - there's a lot of mechanical energy there. Bob
