Adding a 3rd floppy to an old PC SCSI controller

Chuck Guzis cclist at
Tue Apr 1 12:14:38 CDT 2008

Dwight asked:

>  Pardon me for asking but are you sure the chip can provide
> enough drive? Most floppies expect to terminal with about
> 150 Ohms. That is more than an unbuffered output of most
> chips are designed for.
>  Just asking?

Good point--but the datasheet states right in the opening remarks 
that it can be connected directly to the disk drive with its own 
internal 48 ma buffered output and schmitt inputs.  The datahseet's 
easy to find on the web--several places have it.

Here, for example:

I like the DP8473--unlike the Intel 82077AA and later chips, it still 
knows how to do FM correctly--and will even write 128 byte sectored 
MFM, which is something that Intel was ever able to do.  Don Maslin 
and I worked through a pile of FDC chips before we found this one. 

I think there was a Future Domain card (1680?) that brought all 4 
floppy drives out to 2 headers--and I certainly could have added one 
to do that myself for 4 drives, but 3 on one cable was the easiest.


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