Replacing 8" drive with 3.5" drive on CP/M systems

From: Herb Johnson - null account <>
Date: Thu Feb 10 11:40:12 2005

Here are some summary responses to my posts on this thread. I'm working
on how I post to cctech, so these are delayed postings: pardon any
confusion. Also I'm changing my posting address to foil email address
harvesters who troll Web page archives for email addresses. My email
address is coded at the end of this message. - Herb Johnson

Fred Cisin wrote:

> On Mon, 7 Feb 2005, Herb Johnson wrote:
>>> The Epson Geneva PX-8 disks that I have seen used a
>>> 67.5 tpi 3.5" drive (40 tracks per side).
>>> THOSE drives are VERY hard to find.
>>> I have ONE of them. I doubt that there is ANYBODY on this list
>>> other than those who have a PX-8 who own a compatible drive.
>>> "Normal" 3.5" drives, whether "720K" or "1.4M", are 135 tpi.
>>> YES, you could "double-track" it, and introduce track width probs
>>> But, to use that otherwise good idea, consider other 3.5" CP/M
>>> formats that use a "NORMAL" drive.
>> Normal track densities are listed in my Web site document, taken from
> "Normal" track densities.
> The Epson Geneva PX-8 was NOT "normal".
> It's almost as abnormal as 100tpi 5.25".

I've asked Fred for details about the Epson drive, so I can add them to
my archives on my Web site. He may also post our correspondence here. In
any event, it's clear that the Epson drive - actually called the PF-10 I
believe - is not a good reference for a standard.

Ethan Dicks wrote (on cctalk) on Feb 5th:

> Yes, but it's really hard to find 3.5" DD media these days... ask any
> Amiga person.
> If _I_ were designing/changing floppies in modern times, I would
> certainly see about using HD media because I think they were more
> abundant, and they are still available, new, making them available
> further into the future.
> -ethan

A Google search for '3.5" 800K diskettes' provided many sources for
unused old stock, and I think a few new stock. Prices ranged but some
were quite low.

But please note: I'm talking about whether HD media and drives will
ACTUALLY WORK RELIABLY on the "non-modern" hardware I described.
"Convenience" and "abundance" does not help if the disks don't work. As
for buying DD 3.5" diskettes, I understand the issue, I sell such disks
myself. But A Google search on 3.5" DSDD diskettes found a number of
suppliers, here and now. In any event, people who use and maintain old
hardware should not be surprised that there may be limits as to how far
they can upgrade, yet maintain compatibility and reliability.

Moving from 5.25" drives to 3.5" drives is less of an issue, almost all
of those earlier systems supported double density (MFM). But not all,
such as the original Osborne 1 with single density, singled sided 5.25
inch drives. There are still other issues with this change, but they can
be mananged for the most part.

A search of old comp.os.cpm archives will find prior discussions of all
this. Some of those discussions are archived on my site as I noted, some
data on drives and media are on my site.

Randy McLaughlin (general posts)

Randy started the thread with his proposal for a "standard". In
correpondence public and private, he has not quite appreciated the
potential problems of single density (SD) and FM format unique to the
oldest systems, versus the use of new HD drives and media. But I think
he appreciates now there will be more than ONE "standard" which would
include a SD scheme of some sort.

My impression is that Randy has not considered that diskette drives and
media are ANALOG devices. That is, the read write electronics, the drive
head, and the media, have responses that depend on data rates
(frequency) and modulation (FM and MFM). Consequently, they have have a
marginal, unreliable response if they are used outside their design
limits. That is, drives and media from different manufacturers can be
DIFFERENT, and possibly inconsistent, in how they respond to any unusual
(non-standard) schemes of data rates or formats. The worst of this is
how HD drives respond to SD and DD formats and data rates from older
systems. Also, response may change over time, as drives and media age.

Of course, a classic issue of design differences, is the fact that HD
and 80-track drives have heads which create thinner tracks. That's why a
disk formattted for 40 tracks on an 80-track drive may be UNRELIABLE
when read on a 40-track only drive. Thinner tracks means poor erase of
old thick tracks; and less signal when the thin track is read by the
"thick" head.

What I've heard over the years about uses of HD drives and media for
"earlier" formatting and controllers, suggests there may be problems at
least in the long term, as well as in the short term. Specifics, when I
come across them, I'll add to my site, and some of that information and
discussion is already there. (Some are posting on this already; as time
permits I may review archives and with permission add more to my site,
referenced at the link below.)

This is why I believe it is better to keep older systems with original
drives and media whenever possible; or at least to make such drive
changes that are CLOSEST to original drives and media. Old drives and
old media are still available. New media and unused old stock are still
available with some searching. USed stock, by my experience, is often in
good shape if stored well and if bulk-erased.

HOWEVER some of the oldest diskettes like 8-inch do fail or perform
poorly - why is not clear to me. And old 8-inch drives can just wear
out, or fail and destroy media. It's a bad feeling when a disk drive
head SCRAPES UP A DISK to bare plastic. So the issue of replacment and
upgrade is a reasonable one.

I'll note that I sell old floppy drives and media, and of course I have
my own old systems, so I have my biases. That's why I try to find
information and data, so others can become informed and draw their own
conclusions, make their own decisions.

Herb Johnson

Herbert R. Johnson, voice 609-771-1503, New Jersey USA
<a href=""> web site</a>
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Received on Thu Feb 10 2005 - 11:40:12 GMT

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