Early 3.5" Floppy Drives - test-drb@ccmp.vtda.org

15 Dec 2005

...

Subject: RE: Early 3.5" Floppy Drives
   From: "Chuck Guzis" <cclist at sydex.com>
   Date: Thu, 15 Dec 2005 13:29:46 -0800
     To: cctalk at classiccmp.org

n 12/15/2005 at 3:21 PM Allison wrote:

 >
>Subject: RE: Early 3.5" Floppy Drives
>   From: "Chuck Guzis" <cclist at sydex.com>
>   Date: Thu, 15 Dec 2005 11:30:29 -0800
>     To: cctalk at classiccmp.org
>
>On 12/15/2005 at 1:18 PM Allison wrote:
>
>>;) your assumption is double density.  8" SSSD is not that fast.
>>I never said formats were the same or even dive interface only that
>>the data rates fly.
>
>Nope.  I'm just going by the 765 data sheet:
>
>"Pin 19 - CLK - Single-phase 8 MHz (or 4 MHz for mini-floppies) squarewave

clock"

IOW, if you supported 3.5  DD (or SD) floppies, you weren't going to be
able to do an A1 8" floppy without changing the clock. 
;)  You know not the part you speak of.  Question, what it that clock used 
 for?  Hint data rates are NOT tied to it.  

....unless you count WRITING :) -- or is it your contention that FDC's not
be capable of writing data?.  AFAIK, neither NEC nor its licensees has
changed the relationship between the write clock and the 4 or 8 MHz clock
input.  
Save for wrtclk controls that pin21 not the clock on pin19

...
  Could a 765
running off of a 4MHz clock, given the proper data separator read 8" A1
diskettes?  Maybe, but there are some other things tied to the
clock that might have an effect, such as the length of the VCO sync-up
period.  Could it write or format them?  No way--it's just not built that
way. 
It is built that way as the clock supplied on Pin21 is the write clock 
and the RDW pin22 is the read clock.  I've done it, not by plan but by 
error.  Ran well enough but when playing with step rates and heal load 
times the they were off by *2, oops!

The format is controlled by counting the writes.  The VCOsync is timed off
of the pin19 clock most cases even at double the length it works as that 
was the difference between the 765 and the 765A.  Not an issue for 8",
sometimes a problem for dense formats on 5.25" (10sectors of 512bytes),
big issue for 3.5" though there was the 7265 tuned for that (no index gap
written on format).

...
  Not even close
765 is a wholly differnt animal.  The Read operation needs 
RDW and the write must have WC.  Both are independent of the chipclock. 
Where be this WC pin on the 765 you speak of?  I don't see it.  Heck, I
don't see it on the 179x, either. 
Look for PIN21, The current data sheet has WCK.  Older ones have WC.

The 179x write is clocked off of main clock with a divisor selected by
fm or MFM so 179x pin24 not only drives the internal uengine its direct
control of the write shift logic.

there are some very distinct and fundemental differnce between the WD
177x, 179x and the 765.  the most basic is that the 765 has both head 
select and unit select logic and also handles Ready and Fault. 

...
 IMOHO, I note that it's the newer smaller drives,
not the 8" drives that
step faster, so, using your logic, it'd be the 8" drives, not the 5.25"
that needed the slower clock. 
Some do. Some don't.  Depends on the era, as 765 is now 25 years old. 
A lot of drives have come and gone a few lasted a while.  CDC 8" 
drives were pretty happy with 4mS step rates but buzzed like a 
banshee at 6mS. 

I have the distinct advantave of having supported the part in the field 
 as "factory" for over two years and playing backup for the product 
engineer reponseable for the part for another two years while at NEC.
Any question I didn't have answers to were likely propritory.  So 
between having all the docs and a tube of them it's been the FDC of
choice since 1980 for me.  When I have the 6809 CUBIX system going
that will make the 8th unique design using the part (765A) never 
minding having used the 9266, 37C65 and 37C665.

Allison