searching for replacement for 1793 FDC

[Dwight K. Elvey]

>From: "Gooijen, Henk" <GOOI at oce.nl>
>
>> 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.
>
>

Hi
 The 2791/93 are a more flexable choice than the 2795/97.
>From my understanding of the spec sheet, the 2791/93 can
do both 5.25 and 8 inch without changing external parts.
The 2795/97 requires different clock frequencies to do
this.
 You give up the side select but as was mentioned before,
I suspect that one could double up the direction bit
to also do side select or just have a port to do that.
Dwight