Re: Apple ][ plus 13 sector! Problems...

[mjmahon@aol.com (Michael J. Mahon)]

Bill Garber wrote:

>"Bryan Parkoff" <bryanparkoff@nospam.com> wrote in message
>news:40f997a8$0$74186$39cecf19@news.twtelecom.net...
>: Bill Gerber,
>:
>:     It is what I understand that there are two different
>sequencer logic
>: PROMs.  First is to use 9 bits that it can only have one zero
>while second
>: is to use 10 bits that it can only have two zeros.  It is why
>13 sectors use
>: 9 bits while 16 sectors uses 10 bits.
>:     Do you have 13 sector PROM that you can spare?
>
>The firmware can handle how many zero bits to look for.
>State machine only does what it's told to do.

Actually, Bryan is right, the state machine PROM is different
to allow reliable reading of disk nibbles with two consecutive
zero bits.

I don't follow his "9-bits...10-bits" points, since both controllers
have always used 8-bit nibbles on disk.

FWIW, the change from 5+3 coding to 6+2 coding, allowed
by increasing the number of valid nibbles to 66 (>64), is what
permitted 16 sectors to fit on a track instead of 13.

The new state PROM can reliably read 13-sector disks,
but the old PROM cannot reliably read 16-sector disks
with normal speed tolerances.

-michael

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