[Coco] [COCO] MAME FDC Issue that can cause corruption

[William Astle]

On 2017-08-01 11:03 AM, Gene Heskett wrote:
> While it not universally done, William, leaving the motors on makes for
> much quicker disk to disk copies on the real hardware, and I'm rather
> positive that it was not ever a problem on the real hardware running os9
> or nitros9. I can't testify about rsdos/basic, never used it enough to
> see an error if there was one, but a backup operation could change the
> access lines as indicated on the front of the drive just as fast as the
> nitros9 drivers do.
> 
> An led tally  on the DS lines would prove the point, as would a dual
> trace scope set for slow sweep. Any overlap would not be the desired
> thing.
> 
> So this does have the smell of a MAME problem.

Yes, enabling all the motors on is much faster when going between 
drives. I know for a fact that DECB just changes the drive select bits 
in that case. It actually checks for the motors being on and skips the 
"wait for motors up to speed" delay. It doesn't consider the drive 
selects - only the motor bit.

Of course, that didn't stop Radio Shack from selling broken hardware. 
Later "drive 1" expansion units for the FD502 system were hard-wired to 
tie the motor enable to the drive select which was definitely not 
helpful. (I know this because I had such a unit and the jumpers were 
soldered. Took a while to figure that one out.)