searching for replacement for 1793 FDC

[Holger Veit]

Gooijen, Henk wrote:

>>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
>  
>
Hi Henk,

What appears to make the design on that page a bit difficult is the 
presence of the CPLD, but this is not
really required, unless you need full SS30 compatibility, and at the 
price of losing some flexibility in setting
modes; the 2797 datasheet shows a really simple application circuit.

>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 2797 is available from www.bgmicro.com, and they have no problems to 
sell to Europe; I have ordered
several times from them already, and I guess if it works for Germany, it 
will also work for the Netherlands; surely it is
not useful for a single 2793, but they might have more attractive 
gadgets for the electronics enthusiast to reach a
minimum order limit (had never problems WRT).

>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 ...
>  
>
They are for precompensation, and the datasheet explains how to set them 
- one better has an oscilloscope for it, but
I guess this won't be better for a 1793 either.

Regards
Holger