Needed: 1 IBM 8" alignment disk. - test-drb@ccmp.vtda.org

30 Nov 1999

If the drive is "intelligent" enough to perform writes, i.e. if it has a
local data buffer, then it's likely to have a master oscillator.  Since it
has adjustments for "window" (probably related to FM clock recovery) and
"gap" probably limiting the write-turn-on/off gap lengths, i.e. the capture
window, allowing it to determine when it failed to "see" and address mark,
I'd suspect that those have gotten tweaked out.

Dick

-----Original Message-----
From: Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Tuesday, November 30, 1999 2:17 PM
Subject: Re: Needed: 1 IBM 8" alignment disk.

...
 > >They'll be a master clock, probably also
used for writing. Check this
> >with a 'scope or frequency counter. If it's incorrect, find out why.
> >
>
> Generally, there isn't a master clock.  Among the drives I've been
working
...

Not on a raw drive, sure. But this is a drive/controller combination, and
it appears that at least some faults are in the controller section.

Somebody's going to prove me wrong, but I've never seen a _controller_
without some kind of master clock.

> on over the past months, none had onboard oscillators with the exception of
...
 > the microprocessor-controlled Mitsubishi. 
That's why there are one-shots
> and the like.  The writing is accomplished by using both outputs from a
> flipflop which is toggled by every positive edge on the data stream which is
...
   generated on
the controller.

Yep, sure for a plain drive....
[And I agree with the PLL comments that I've deleted -- very few _drives_
have a clock recovery PLL, but a lot of controllers do!]

-tony