Re: Floppy reading at flux level, WAS: Re: Tips for making Twiggy images

From: Natalia Portillo <claunia_at_email.domain.hidden>
Date: Tue, 17 Feb 2015 16:46:54 +0000

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Hi Ray,

El 17/2/15 a las 10:48, Ray Arachelian escribió:
> I suspect the GCR used on the Lisa is identical, or nearly
> identical to that of the Apple ][ series and early 400K Mac,
> because it uses the same or nearly the same IWM. There's a
> dedicated 6504 CPU with ROM and shared RAM.

We can always ask Woz.

> The hard part with Twiggies is that they aren't the same as 5.25"
> floppies, the track placement may be different, the density of the
> data for sure. So the question is, is it possible to use off the
> shelf 5.25" common IBM PC high density floppy drives to read Lisa
> Twiggies with some controller based on ARM or otherwise that's
> capable of decoding GCR?

The discferret team though that is possible.

> I agree that since modern CPUs are fast enough (heck, even the
> lowly 6504 running at 1MHz is fast enough) to decode GCR, you'd
> want to access the floppy drive itself as an analog device and
> capture and decode flux transitions.
>
> Much of the GCR details are shown in the Lisa Hardware Guide 1981
> and 1983, plus disassemblies of the I/O ROM are available as well.

> I'm not 100% sure but I believe the original Inside Mac manuals did
> provide some details as to the workings of the IWM.
>
> I didn't actually emulate the full 6504 subsystem of the Lisa,
> rather, I emulated the interface it provides, so I didn't need to
> deal with the details of the GCR format. Instead, I simulated the
> RWTS routines. I also captured and detected attempts to execute
> native 6504 code by Lisa software, then hand disassembled them and
> tried to figure out what they did.
>
> Some Lisa OS will use this to detect if a 400K floppy is read
> only, others will try to check for the presence of the floppy, I
> guess these were oversights that weren't included in the I/O ROM.
> So far none of the Lisa software I've seen had any funky copy
> protection schemes such as writing extra tracks, hack tracks,
> errors, sectors, sync marks, or using the wrong speed, - they all
> seemed to be locked down to the particular Lisa's serial number in
> the VSROM. (It doesn't mean no such protections exist, of course,
> but rather that I've not seen them.)
>
> The other question is, is it worth building such a device? Twiggy
> media

Yes, if we build it for all media, not just twiggy.

> are very rare, and likely someone who owns such media likely owns a
> Lisa 1 or likely has sold it to someone who owns a Lisa 1 as Lisa
> 1 collectors are very fierce in completing their collections. And
> likely if those Lisa 1 Twiggy drives work, BLU has been shown to be
> able to image those floppies. It may be worth building an external
> reader just for the challenge of it, but once you've captured most
> of the Twiggy media out there (commercial software such as LOS,
> LPW, and the apps) and published it on BitSavers, there's not much
> use for the device.
>
> I'm not saying not to do it, but rather that it's a very small win,
> when folks on here already own Lisa 1s and with James's excellent
> BLU have been able to image Lisa 1 floppies successfully.
>
> The one area where it would be very useful is in the case of
> damaged Twiggy media - providing the ability to read at various
> speeds, and fine step the stepper motor to access and recover data
> that's no longer readable by a real Twiggy is worth it..

There's a huge possibility that some unique disk appears that's too damaged for BLU. Not to say, I really want to know where are the rest of the tag bytes.

> Or perhaps if one is able to build a replacement Twiggy drive using
> off the shelf 5.25" floppy drives with a controller attached to
> handle the differences, it would provide a viable modern Lisa 2 ->
> Lisa 1 conversion kit, or a replacement for damaged Twiggy drives.
> Of course the drives would need to be modified to provide a
> software eject mechanism, but a modern replacement for Twiggy
> drives that would fit in a Lisa chassis along with a replacement
> faceplate, I/O ROM in a kit would be a very good collector's toy.

That's also possible. http://hxc2001.free.fr/floppy_drive_emulator/

Regards,
Natalia Portillo

>
> On 02/17/2015 05:04 AM, Simon Claessen wrote:

>> 
>> 
>> On 16-02-15 20:17, Natalia Portillo wrote:
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>>> 
>>> Hi Simon,
>>> 
>>> I'm a programmer, I need an electronics.
>>> 
>> 
>> 
>> 
>>> Anyone here up to the challenge?
>>> 
>> 
>> 
>> yes. :-) im an electronics and need a programmer.
>> 
>> You can even use a beaglebone black as a platform. But
>> electronics is not the problem. Detecting the right format of a
>> floppy is. And knowledge of different floppy formatting
>> algorythms like FM, MFM, MMFM, GCR and such.
>> 
>> The point where kryoflux got it right is that the decoding is
>> done on the host computer and the device merely is a dual channel
>> digital sampler (clock and data pulses)
>> 
>> i can cook up a decent adapter board in no time to stick as a
>> cape on the beaglebone.

>
>

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Received on 2015-02-17 17:17:53

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