3.5" floppy disk revisited on the UNIX pc ...

[C M Votava]

In article <695 at icus.islp.ny.us> lenny at icus.islp.ny.us (Lenny Tropiano) writes:

>After coming back from the Trenton Computer Festival last weekend, both
>with 720K 3.5" floppy disk drives (for $30 a piece), we decided to install 
>them in our UNIX pc's.
>
>Now how did we manage to get both drives on the UNIX pc at once, since the
>hardware only supports one ...  Well it's sorta a hack, but basically it's
>a mechanical switch (SPDT) that switches the FDRIVE0* signal (pin 10) between
>the 5.25" drive (360K) and the 3.5" drive (720K).

I did this "enhancement" to my unixpc very recently too, but solved it in a
little different way. First, I simply replaced the existing floppy ribbon
cable, with a stock IBM PC floppy ribbon cable (the one with connectors for
2 floppys, without the "twist" in it). Next, I moved the drive select on the
5.25" floppy from drive select 0 to drive select 1, and set the 3.5" floppy
drive select to drive select 2. Then, all I had to do is wirewrap in a switch
off of the connector on the 3.5" floppy that connects drive select 0 to either
drive select 1, or drive select 2 to engage either drive.

The next step, is to modify Emmet's MSDOS programs to access the higher
density drives. In the source he provides, he includes the init.c routines
for both the unixpc, and IBM-PC's so it was a simple matter to copy the
appropriate values from one to the other. I now have the "mdir" command
working fine off of the 720K disks! However, when I try to use one of the
programs that actually go out and try to access the file (like mtype) all I
get back is garbage. Obviously, there is a problem in finding the start of
the file using the pointer in the FAT, but I haven't had time to figure it
out yet. Anybody want to throw me a hint before I dive in all the way?

The final step will be to get the DOS-73 code to handle the 720K disks, that
will be nice!

-Craig Votava

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