Path: utzoo!attcan!uunet!husc6!think!ames!pacbell!att!cbnews!mvadh
From: mvadh@cbnews.ATT.COM (andrew.d.hay)
Newsgroups: comp.sys.att
Subject: Re: 3B1<->3B2
Keywords: what compatibilities?
Message-ID: <2871@cbnews.ATT.COM>
Date: 4 Jan 89 12:30:49 GMT
References: <7047@ihlpf.ATT.COM> <598@whizz.uucp> <2328@cuuxb.ATT.COM> <379@ssbn.WLK.COM>
Reply-To: mvadh@cbnews.ATT.COM (andrew.d.hay)
Distribution: na
Organization: AT&T Bell Laboratories
Lines: 54

In article <379@ssbn.WLK.COM> bill@ssbn.WLK.COM (Bill Kennedy) writes:
-->In article <2328@cuuxb.ATT.COM> fmcgee@cuuxb.UUCP (Frank W. McGee) writes:
.
.
.
-->So far so good, but what Frank didn't say was that in order to fit 1.2Mb
-->on an AT style diskette they muck with the spindle motor speed.  It's my
-->understanding that the 3B2 uses a 96tpi drive like the AT and that the
-->reason that only 720K will fit is because the spindle motor speed remains
-->constant, thus only twice the capacity of a 40tpi drive.  On the outer
-->tracks there's some magnetic real estate that can be used by an AT type
-->drive that just isn't used because the rotational rate is constant.  If
-->it slows down, then more bytes can be fit on it.

as far as *I* know, the formats are as follows:
single density:	125Kb/s (fm);
double density:	250Kb/s (mfm);
quad density:	500Kb/s (mfm) with spindle speed raised from 300 to 360 rpm
(just like 8" floppies -- remember 8" floppies?).
if you work out the math -- for a 160 track drive, this gives ~1.6Mbit
unformatted.

-->This is nearly as much of a question as it is a statement.  I know, for
-->example, that "vanilla" 96tpi drives aren't compatible with the AT, nor
-->can you expect to get 15 sectors per track on the inner tracks of a
-->"vanilla" 96tpi drive.  Doesn't the AT type drive slow down on the inner
-->tracks to keep bits per inch about the same as on the outer tracks?

no, it *speeds*up* on *all* tracks.

-->>is no amount of software that can be written that will allow a Unix PC
-->>to write a 720K disk).  However, if you've done the hardware mod to put
-->>a 1.2 MB drive in all you'll have to do is hack the device driver to
-->>allow the 720K format).  The 720K format is double-sided, quad-density,
-->>at 40 TPI if I remember right.

i had thought the 3b2 format was 80 track double density, not 40 track quad
density.  a 3b1 could do this with an 80 track drive, at the expense of 40
track compatibility (since it doesn't know about double-stepping).

i didn't know any 40 track drives were quad-density rated.
to do quad density on the 3b1, you need to change the controller clock.

-->To be a hopeless nit picker, is it not 40 cylinders on 48tpi media and
-->80 cylinders on 96tpi media?

yes -- though the earliest 96tpi drives had 77 cylinders, like 8" drives
(remember 8" floppies?).

-- 
Andrew Hay		+------------------------------------------------------+
Holistic Specialist	| I will design a computer for you, so powerful that   |
AT&T-BL Ward Hill MA	| organic life will form part of its operational matrix|
mvuxq.att.com!adh	+------------------------------------------------------+