searching for replacement for 1793 FDC

Gooijen, Henk GOOI at oce.nl
Thu Sep 1 04:36:55 CDT 2005


> -----Original Message-----
> From: cctalk-bounces at classiccmp.org
> [mailto:cctalk-bounces at classiccmp.org]On Behalf Of Dwight K. Elvey
> Sent: dinsdag 30 augustus 2005 19:47
> To: cctalk at classiccmp.org
> Subject: RE: searching for replacement for 1793 FDC
> 
> 
> >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


Thanks for pointing that out Dwight!  I overlooked that!
Only the 2791/93 has an *internal* clock divider circuit needed when
you control either 5.25" or 8" drives.
For the 2795/97 this means additional external circuitry.
I have in my design an octal latch to control some things, so density
can be controlled with one bit from the latch.
All in all, I will make my design with the (available) 1793. I only
have *one* trimpot adjustment, the 279x has *two*.
I will take the +12 from the power supply. The 7660 is a good suggestion,
but increases the chip count by one, unnecessary.
Ok, one wire added instead :-)
BTW, my design should work with 8", 5.25", and 3" proven years ago, so I
expect that 3.5" will work too ... time to heat up the soldering iron!

  thanks,
- Henk, PA8PDP.




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