searching for replacement for 1793 FDC

Gooijen, Henk GOOI at
Tue Aug 30 07:22:54 CDT 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 
> (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 ...

- Henk, PA8PDP.

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