From: tom.hornberger@verizon.net   
      
   Joe Zwerko wrote:   
      
   > "Joe Zwerko"  wrote in message   
   > news:Ebqdnb3BCNOSnjjVnZ2dnUVZ_t3inZ2d@comcast.com...   
   >> Probably a stupid question, but here goes...   
   >>   
   >> While waiting for more parts for my double-density project (from Bulgaria   
   >> of all places) I decided to go ahead and get 40-track SINGLE-density   
   >> double-sided working.   
   >>   
   >> Well, so far, so good.  I can format and use a double-sided disk on any   
   >> of   
   >> my several spiffy "almost-new" Shugart SA455 drives.  The problem is, my   
   >> OS'es are all on old 35-track SD SS diskettes.  I've tried everything I   
   >> can think of to backup to a bootable SD DS floppy, but it doesn't work.   
   >> I'm using LDOS.  Yeah, yeah, I've done "backup :0 :1 (sys,inv)" and it   
   >> copies a lot of stuff, but the resulting diskette isn't bootable.   
   >>   
   >> Has anyone tried this?  Is NewDOS better?  Is there a secret incantation   
   >> you could let slip?   
   >>   
   >> pjb   
   >   
   > I tried sending this a few hours ago with some photos, but it disappeared   
   > into a black hole...   
   >   
   > Today I finished the double density adapter prototype.  It's on a   
   > solderless   
   > breadboard and looks pretty questionable, but it works fine.  It's the   
   > Tandy design, with vintage WD parts obtained from eBay.   
   >   
   > I used a ribbon cable about 9 inches long to connect to the WD1771 socket   
   > inside the EI.  Of course this caused a few minor electrical problems from   
   > signal reflections.  The only change I made to the Tandy design was to add   
   > a   
   > 74LS244 to drive the DIR, STEP, WD, and WG signals back to the EI.  I was   
   > fairly surprised that I didn't have to add terminators on any of the   
   > signals from the EI.   
   >   
   > I'm not sure if I'm going to fab a PCB for this; I'll probably just put it   
   > on a perf board and spend a week soldering all the little wires, then   
   > mount it inside the EI case.   
   >   
   > First, though, I want to try something: does anyone know of any reason why   
   > the WD1771 is needed when there's a working WD1791 (well, the equivalent   
   > MB8876A in my case)?  As far as I can tell from the chip specs, the   
   > 1791/8876 is compatible (functionally compatible, not pin compatible) with   
   > the 1771 when the DDEN* (double-density enable) pin is inactive.  It'll be   
   > using the external data separator (which the 1771 does not in this   
   > design), but the WD1691 provides an internal divide-by-2 when DDEN* isn't   
   > active. But surely there's some reason why all the DD adapter types I've   
   > seen kept the WD1771 on board.   
   >   
   > pjb   
      
   The WD1771 can generate a couple of Data Address Marks(think that's the   
   correct term) that can't be generated by the WD1791.  Of course there's at   
   least one of those Marks used by TRSDOS to mark one of the sector types.   
   The MOD 3 & 4 have the WD1793 (I think), that's why a Mod 3 or 4 can't   
   properly copy Mod I disks.   
      
   HTH,   
   Tom   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)