Date   : Wed, 10 Oct 1990 11:48:40 GMT
From   : sung@mcnc.org (Wayne Sung)
Subject: converting 8 inch to 3.5 inch

There have been some discussions in this group recently about using HD floppies
to replace 8 inch drives. I tried this using a 3.5 inch HD drive and here
are some
early results.

Hardware:
In general there are no hardware difficulties. If you looked at the pinouts
of an 8
inch drive and a 5.25 inch drive you will find that many of the pins line
up one for
one. I happened to have an adapter board that converted between the two
size ports
so I did not have to make a cable. The drive that I got has an adapter
to go from
the 34 pins to a 34 card edge but either would have been ok. Two signal
lines had to
be accounted for. The 8 inch drive has a drive ready line which the smaller
drives
do not. In my case this line is sensed by the bios but the easiest way
out is simply
to keep it grounded. There is also a signal out of the 8 inch drive indicating
that
a two-sided disk is inside. Since there are no single sided HD disks, this line
could also be grounded if I was to replace all my 8 inch drives. In mixing
single
side 8 inch and double side HD, it's trickier because the line is used
during format
sense. I presently have a jumper which has to be removed for single side
disks. One
interesting sidelight: the last set of 8 inch drives I had been using cannot
step at
3 ms rates. These 3.5 units can, and are actually much quieter stepping
at 3 ms than
at 6 msec.

Software:
The formatters that I have don't exactly behave right. For one thing, they
are set up
for 77 tracks which wastes some 50-60k of space. They also don't seem to
be able to
format up to the highest sizes (presently 1.2M on the 8 inch system). I
suspect the
track layout has to be redone, since with the much smaller circumference
there is less
room for pad bytes and the 8 inch format has a lot of pad bytes. I did
not have any
difficulty reading the 1.44M size that had been formatted on another machine.
Of
course the DPB will also have to be adjusted to account for the extra tracks
when I
get to use them.

If anyone would like more info let me know.