it's well to remember the "GCR" stands for group-coded-recording, which can
mean a lot of things, and that it's a form of "RLL," or run-length-limited"
encoding. Both operate in a way that generates, typically, more transitions
in the aggregate, than the original (NRZ) data stream, but locates the
transitions propitiously in a way that limits their density so that it remains
within the head-media combination's capability to resolve those transitons
accurately enough to allow data recovery. That leaves LOTS of room, and it's
in this room where both the Apple scheme and the Commodore scheme live. My
Apple-targeted HDC from back in the early '80's used ANSI standard
group-codes, which, certainly isn't the only uesable scheme one could use, but
it was established, and I could hand it to someone to build a lookup table for
me, which is what I did.
Woz did something different, and so, probably, did the Commodore guys. To
qualify for group-coded-recording, all one required was a unique mapping from
one pattern to another, and and back again, and both schemes accomplish that.
The purpose of the scheme is to rearrange the flux transitions so the system
can tolerate them, yet allow more of them to be represented within a given
linear distance of head travel with respect to the media.
Dick
----- Original Message -----
From: "Ethan Dicks" <erd_6502_at_yahoo.com>
To: <cctalk_at_classiccmp.org>
Sent: Wednesday, May 15, 2002 10:46 AM
Subject: Re: [CCTALK] Commodore 1541 Drive
>
> --- Pat Finnegan <pat_at_purdueriots.com> wrote:
> > I'm trying to get one, and wondering what success people have had with
> > connecting them to a PC using "X-Cables" and "Star commander" to transfer
> > data ( http://sta.c64.org/sc.html ).
>
> I have a homemade cable that is the model *before* the X cables were
> devised. I keep a 486 around for reading 1541 disks.
>
> > Also, is there any chance of making
> > a 1541 read an Apple ][ disk? I know they both use GCR encodings for
> > their disks, and it'd be really useful if I could read disks on my PC.
>
> Not as far as I know. I think the lowest bit density on the 1541
> is identical to what the Apple ][ uses over the entire surface (it's
> 17 sectors of 256 bytes on the 1541, vs 16 on the Apple ][, but
> due to header/trailer differences), but I do not recall a single
> program or trick to read an Apple disk in a 1541.
>
> I was a beta tester for the Spartan Mimic, the expensive and unpopular
> Apple "emulator" for the C-64 (co-processor is more like it). One of
> the boards that came in the box was a card that sat inside the 1541,
> between the drive mech and the 1541 mainboard. It had lots of relays
> and stole the drive mech out from under the 1541 board. That was
> the only Commodore drive I ever saw read an Apple ][ disk, and it
> still needed an Apple controller to do it (there was a 20-pin connector
> on the Spartan drive board).
>
> Among other differences (smart vs dumb peripherals), the 1541 depends
> on a certain header design - there's an 8-input NAND hooked to a shift
> register on the input side, and the Set Overflow pin of the 6502 on
> the other. When a bunch of 1s go streaming past the head, this logic
> kicks the processor out of a tight loop where it branches to itself
> while the overflow bit is clear. This, coupled with precise timing,
> lets the 6502 in the drive know when to grab the data from the sector
> header. I can't describe how the Apple ][ state machine PROM works,
> but it is its own flavor of beast, resembling nothing else in the
> home-computer arena. I'm not surprised that nobody ever got things
> to read in other brands of computer.
>
> > Also, does anyone know anywhere I could get a Compaticard (and any
> > necessary drivers to make it useful)? I've looked on ebay a few times now
> > and still haven't seen one...
>
> Can't help you there.
>
> -ethan
>
>
>
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Received on Wed May 15 2002 - 13:43:15 BST