FDCs (was: MITS 2SIO serial chip?

From: Allison <ajp166_at_bellatlantic.net>
Date: Tue Dec 18 15:31:12 2001

From: Fred Cisin (XenoSoft) <cisin_at_xenosoft.com>


>Ah, Allison, where have you been all of my life?


Ah, right here.

>If you would have told me that 15+ years ago, you would have saved me LOTS
>of time and effort putting tape over index holes, making cables with
>switches to interrupt the index signal, etc.


;) and if you havd called NEC during the '79 to '83 time frame you might
have gotten me there. One trick for the short index or late index problem
of most 1793 formats is solved by using a oneshot that is a delay of index
around 98% of rotation time, once it fired the resulting pulse looks earlier
and the index window now has the right time. That makes it easy to wind out
the VCO sync delay in the 765.

>With a 765, if I want to read a sector from the second side of most Kaypro
>disks, I need to feed it a value that matches the WRONG value that's in
>the sector header for the H field. With WD, that field can be ignored.


Ah yep. I did a two sided to match an oddball that had sectors 1 to 18
on the top and 19 thru 36 on the bottom. no problem.

>Raw v formatted track read has advantges and disadvantages. The 765
>approach is very handy for reading a bunch of "normal" sectors. The WD
>approach makes it possible to read MFM that does not follow
>"normal" sector header standards, such as Amiga, or some TRS-80 address
>marks.


Yep, then again depends on what your doing too. If you building a data
recovery machine a 1793 may be it. Then again you'll need a 1771 too
as there were 1771 formats that were unreadable by the DD 1793. If you
building a virgin CP/M machine that needs to read a smaller subset of
media... 37c65 or later is far cheaper and easier.

>Personally, I would have preferred the "raw" approach of the WD, but I
>make no claims to be representative of the marketplace.


It's only useful for the special cases. For everything else you wnat a
track
or cylinder without the junk.

Allison
Received on Tue Dec 18 2001 - 15:31:12 GMT