From: lowen@lorc.UUCP (Lamar Owen)
Newsgroups: comp.sys.3b1
Subject: Re: TEAC Combo 3.5 & 5.25 FD drive
Date: Tue, 06 Oct 92 08:33:27 EDT
Organization: LORC Enterprises

adh@cbnewsi.cb.att.com (andrew.d.hay) writes:

> In article <91099@rphroy.ph.gmr.com> rhaar@albert.cs.gmr.com writes:
> []
> "     But *some* 1.44M 3.5" drives can be jumpered to operate 
> "like 720K drives.
> 
> 1.44M drives are DSQD 80TK; 720K 3.5" drives are SSQD 80TK, not DSDD
> 80TK.

Bzzzt.  1.44 meg drives are 80 track, double sided, 500Kbps data rate, 300
RPM drives.  720K drives are 80 track (135 TPI), double-sided, 250Kbps
data rate, and 300 RPM.  The 500Kbps rate is commonly referred to as High
Density (HD), while the 80 track (135 TPI) is commonly referred to as
Quad density.  This usage stems from the history of the 5.25 format, which
started out at the same density as 8 inch drives (48 TPI).  I forget the
density figures in Flux changes pre degree, but they are available.  The 5.25
format started life as single-density (the common name for 250Kbps FM recording)
single-sided, 48 TPI, 35 track.  This provided about 90K of storage.  The next
improvement was an encoding method called MFM, which, due to its near doubling
of recording space (the actual amount of total space per disk was doubled by
increasing the number of tracks to 40 at the same time) was commonly referred
to as double density.  The actual numbers are as follows: a single density, 35
track disk on a TRS-80 (a good example) contained 10 256 byte sectors per track
with 35 tracks--89,600 bytes of space.  A double-density disk had 18 256 bytes
secotrs per track with 40 tracks--184,320 bytes--more than double, even though
the number of bytes per track only went up from 2560 to 4608.  Now, this is 
using the TRS-80 format....  Soon, double-sided drives were introduced, giving
the capability of storing 180K in single density and 360K in double density.
Well, the system designers, after checking with the media vendors, decided
to go the next step and double the track density as well.  Thus, the old
quad density drives.  A good example of these drives are the Mitsubishi push-
latch units found on the Tandy TRS-80 Model 2000.

Well, IBM was at this point in time designing the AT system.  It decided to go
the next step, and double the data rate as well.  However, the media available
at the time was simply not capable of storing the 1.44 MB available with an
80 track, DS, 500Kbps, 300 RPM drive, so they pulled a fast one: the 1.2 MB
drive shipped with the IBM AT used 80 tracks, DS, 500Kbps, and *360* RPM. Note
that this gives the 1.2MB 5.25 drive the same format as an old 77 track 360 RPM
8 inch drive, since the 8 inch standard was 500Kbps.  IBM cut its teeth on the
standard 8 inch format (in fact, IBM originated the format in its 3740 terminal
cluster controller).  The media manufacturers were able to ship media capable
of 1.2 MB, and the rest is history.

The 3.5 inch format, in its single and double sided incarnations, was originated
by Sony.  A single sided 3.5 with 80 tracks and at 250Kbps (MFM encoding) was
capable of 360K.  The technical challenge to go double sided was, of course, the
physical size of the heads on the top side of the disk.  The double sided drive
did come out, and the result was the industry-standard 720K disk (which a 3B1
and an Apple Macintosh will both format happily to 800K by reducing the inter-
sector gaps, changing the interleave, and adding a sector at the end of each
track).  The 1.44 MB 3.5 was simply better media, a smaller head gap, and
a data rate increase to 500Kbps.  The new 2.88MB disks are, predicably, using
a 1Mbps data rate, MFM encoding, and special barium ferrite media, along with
a much smaller head gap.

> Andrew Hay


--

Lamar Owen, Systems Consultant, GE Lighting Systems, Hendersonville, NC
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