On Tue, 14 Mar 2017, Brad H wrote: It would be possible, but a bit of work to make them. A lot depends on whether you want to make replicas, or compatible. Jobs wanted a closed system with significantly increased difficulty for after-market supply of common items. The bizarre Twiggy drive system did not provide improvement over 96tpi double density ("quad") standard systems. A long time ago, Eric Smith explained to me the details about them (he could probably build one): 1) track to track spacing is 62.5? tracks per inch, as opposed to the "standard" 48tpi or 96tpi (or less common 100tpi (Micropois)) That should not be very hard to do, but not trivial. 2) They used variable rotation speed, from 200RPM? to 350RPM?, to make less variation in the FCPI (flus changes per inch) Relatively trivial, for a drive to be used on something other than the original Lisa, as the rotational speed could be left constant, and vary the data transfer rate. 3) Instead of a "normal" pair of facing heads, they wanted to keep single sided heads, with their felt pressure pads, so they went with that strange double slot, providing unmatched opportunities for always leaving a thumbprint on the media. That could be easily worked around, by simply using conventional double sided head, and maybe, in some cases, losing half a revolution waiting for the desired sector to come around. Very few pieces of software (copy protection) rely on the relative timing of the two sides. Since the Twiggy heads are connected to the same mechanism, but on opposite sides of the spindle, when one head is hubward, the other one is rimward, so for most purposes, switching to a conventional head system would provide improvements in speed. The optimum speed method for reading a Lisa Twiggy would be to stick with one side, reading all tracks, and then when at the last track, the other head would be in position to read the first track of that side. That is not the same as what we have become accustomed to, where after reading one track, switch to the other head, and read that track of the other side, and then step to get to the next two tracks. 4) GCR. I don't know the specific GCR pattern used, but that information is out there, or can be empirically determined. 5a) There is an extra hole, for latching the disk in place? or for later plans for a disk changer? 5b) Notch out of one corner to avoid ibverted insertion. 5c) Write protect/enable notch is in a different position. Trivial. (I was amazed that on the "Computer Bowl" quiz show, nobody on Bill Gates' team could remember where the write protect notch on an 8" disk was!) Total capacity was 850K? Media appears to be 600 Oersted (same as 1.2M) You will not be able to use a Western Digital/IBM style of disk controller, due to the GCR, non-WD/IBM sector headers, and maybe the variable speed. BUT, if you manage the track spacing, then it should be possible to read them with a flux transition board, such as Kryoflux. Or, design and build a suitable logic board. There are a couple of ways to manage the track spacing, ranging from analog positioner (Amlyn), extremely crude milling a new "record" for an SA390/SA400, different diameter winding hub for split band positioner, gearing interposed between stepper and positioner, different stepper motor, etc. I know none of the details for doing those, but it does not seem insurmountable. Judging by the eBay response, it looks like a replica (or counterfeit?) would be far more valuable than a usable substitute.

On 2017-03-14 9:13 PM, Al Kossow via cctalk wrote: Only you, Al, only you, would have the patience. --T