searching for replacement for 1793 FDC - test-drb@ccmp.vtda.org

30 Aug 2005

...
  Gooijen, Henk wrote:

 Hi all,
I am looking into the possibility to add a floppy disk interface
to the 6809 Core Board. 20 years ago, so this is OT :-) , I built
a floppy disk interface for my 6800 system using the 1793.
I have ordered some 1793 from BG Micro, but checking the data
sheet of the 1793, I noticed taht the FDC requires +12 on pin #40.
I know the MB8877 is pin-compatible with the 1793, but does *not*
need the +12V.
I failed to locate a seller for the 8877, but I do not know "all"
major part sellers in the US.  JameCo, BG Micro and DigiKey do not
have this part ... somebody knows a good stock of the 8877 ?
I can use the 1793, +12V is "ugly", but the voltage is present as
the +12V is needed for the floppy drive itself!

  thanks,
- Henk, PA8PDP.

  Has it really be the 1793? Or might not be a 2797 design (2797 is
 also available from BG micro) from www.swtpc.com/mholley 
 (New design for SS30 FDC) is more appropriate for "new developments",
 given that it doesn't need that really ugly external data separator 
 logic which always prevented me from building FDC boards (a usable
 separator chip is more difficult to find than the FDC chip, and the
 alternatives with a TTL-monoflop grave is not even more attractive).

 Actually, what is your real problem? You already have 12V for the
 floppy drive, so what prevents you from feeding it into pin #40 of
 the 1793? +12V on an otherwise +5V board is not more ugly than a
 +25V source on an eprommer board, or +12/-5V for 2708/4116 memory
 boards.

 Holger 
Thanks for the reply Holger.
Your remark about the +12 for the FDC // +25 for an EPROM programmer
is correct. I know of the existence of the 2797, but have totally none
experience with it what-so-ever.  The link that you gave is nice, but
I am not sure I can *copy* the design ... Further, I have several 1793
here, so that is cheap for me. I would need to buy the 2797 and ship
it to The Netherlands (sources here are not as good as in the US).
The data separator I use with the 1793 is more stable, and built with
a VCO (LS629, LS393 and LS153).
I will look into the 2797 a bit more, but I guess I'll go with the 1793.
I saw in the diagram of the 2797 that it has also 2 trimpots for the
adjustment ... Not sure yet ...

  thanks,
- Henk, PA8PDP.