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

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.

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?

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 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..

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.

On 02/17/2015 05:04 AM, Simon Claessen wrote:
>
>
> On 16-02-15 20:17, Natalia Portillo wrote:
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA256
>>
>> 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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