Re: Looking for a Micro SCI disk controller

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

"Bill Garber" <willy46pa@comcast.net> writes:
> Combining the 2 answers tells me that the hardware governs
> whether a drive is working at 13 or 16 sector, and adding
> everything else I have heard. Thanks everyone for clearing
> that up.

Specifically, with the Apple Disk ][ controller:

The P5 (341-0009) and P5A (341-0027) PROMs contains 13- and 16-sector
boot code, respectively.  This determines which kind of diskettes can be
booted directly, but does not affect which can be accessed.  It is
possible to boot software from a 13-sector diskettes that knows how to
read and write 16-sector diskettes (but see the P6 notes below).  For
instance, there was a hacked version of MUFFIN, Apple's
13-sector-to-16-sector utility, that worked in the reverse direction.

If you have a P5A PROM and want to boot a 13-sector diskette, you'll need
to either boot the BASICS disk first, or boot DOS 3.3 and run BOOT13.

The P6 (341-0010) state machine PROM can not read 16-sector diskettes
reliably.  It should only be used with 13-sector diskettes.  The P6A
(341-0028) can read both 13- and 16-sector diskettes reliably.  There is
no reason to use the old P6 state machine PROM, even if you're going to
use a P5 13-sector boot PROM.

On the other hand, installing the P5A and P6 together is asking for
trouble, since it would only be able to boot 16-sector disks which it
would be unable to read reliably.

Of course, normally the PROMs were used in matching pairs.  Controllers
that were originally shipped with the P5 and P6 were generally upgarded
to the P5A and P6A at the same time.

Speaking of which, has anyone got any extra P5 or P6 chips they'd be
willing to sell or trade?  At the very least I'd be willing to provide
a P5A and P6A if anyone needs an upgrade.

Failing that, a copy of the P5 and P6 bit patterns would let me burn my
own, assuming that I can find some blank 6309, 82S135, 82LS135,
TBP18S22, 74S471, or 74LS471 PROMs.

Back in 1980 I programmed a double-size PROM (512x8) with the contents
of both the P5 and P5A PROMs, and wired the extra address line to a
switch on the side of my Apple ][+.  IIRC, the PROM was a Texas
Instruments TBP28S42, though the 82S147, 6349, 7649, 74S472, or 74LS472
should also work.  I had to write a little program to reformat the data
because of the way the address lines are arranged on the larger chip.

Eric

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