3 1/2" on the 7300

[a.d.hay]

In article <213 at toylnd.UUCP> dca at toylnd.UUCP (David C. Albrecht) writes:
>Being an inveterate hacker, I've been at it again.  I wondered if the 7300
>could take a 3 1/2" drive since it would make backing up somewhat more
>pleasant and I already use an Amiga so have a supply of the little suckers
>on hand.  They do seem to cost more per MB unfortunately.  Therefore, when
>I saw the sale that good 'ol Radio Shack had on an external 31/2" drive for
>their 1000HX system (for $99, it should still be on), I sprang into action
>and got one.

in this month's byte, computer warehouse has the teac 55G 1.6Mb 5.25"
drive on sale for $89. the orig. drive is a teac 55B.

[	deletions	]
>							  Alligatored in the
>appropriate power lines and tried it.  No smoke but it didn't work.  Noticing
>that the 5 1/4" was selected to D0 while the 3 1/2" was D1, I changed it.
>Voila! It formatted.
>
[	deletions	]
>					Tried formatting a disk with 10
>sectors/cyl ala. some of the 5 1/4" distribution disks, uh uh, no good, hangs
>in formatting.

3.5" & 5.25" QD disks spin at 360 rpm like an 8" floppy, instead of 300 rpm.

>					Now I'm going to have to figure
>out how to hook it up so I can switch between the 5 1/4 and the 3 1/2
>(visions of 36 pin centronics switch boxes dancing in my head).

all you have to switch is the drive select line!

[	deletions	]
>
>David Albrecht


i've just spent last night scrutinizing the hardware reference manual,
to see if it's possible to modify the FD controller to handle QD drives.

if you don't mind losing backward compatibility, it can be done with the
addition of only 1 chip.
you have to double the clock to the 2797, and there's no 2MHz on the
board.  so, divide the 20MHz by 10.(this is the added chip)

the 2793 has a clock divide select pin, to make mixing 5.25" & 8" drives
easy.  unfortunately, the 2797 replaces this with a side-select output
and the clock select (transmit side) isn't tied to the 5"/8" select
(receive side) input.

after all this, you might have to re-adjust the trimmers around the
2797 for the new data rate.

there are setups for ms-d*s that permit 1.2Mb drives to read and write
360K floppies.  for the unix-pc, this would require rewriting the
driver software which is embedded in the kernel and adding some logic.
this would be tough if you couldn't get at the kernel src.
( though if you could, you could easily add logic to allow 4 floppy
drives and 4 16-head hard drives at the same time! )

however, this would allow us to replace our internal drives with a QD
drive without sacrificing compatibility.

---------------------------------------------------------------

i'm investigating qic-40 tape drives with sa-450 (floppy) interfaces
with an eye toward finding one that only needs an iv file to work in
the unix-pc in place of the floppy drive.
these drives are 40 Mb and use dc2000 cartriges.
mountain has one that's selling for $380...

-- 
Andrew Hay		+------------------------------------------------------+
Lunatic By Birth	|	"Don`t try to out-wierd ME, three-eyes!"       |
AT&T-BL Ward Hill MA	|	I can't be bought, but I can be had...	       |
ihnp4!mvuxq!adh		+------------------------------------------------------+