need a HDD

From: Richard Erlacher <>
Date: Tue Dec 12 20:23:15 2000

<stroking beard and clearing throat>

It seems as though, whenever there's a substantial discussion about rotating
memory systems for the computers of yesteryear, the muddled distinctions
between "drive type" as used for describing what is being sought gets mixed
up with "drive interface" and "modulation type" among other concepts. Here,
again, we have a reference to MFM/RLL, both of which are modulation
techniques, both of which were commonly used by SCSI, SMD, and ESDI drives,
among others. Another item that doesn't seem to help, is that the
distiction between physical interface, e.g. ST-506/412 (actually the same,
from all obvious features, though there were some minor differences), gets
mixed in as well. SMD is an interface specification, as are SCSI, SASI, and

A look at the SEAGATE spec's for their various drives, including those made
by CDC before SEAGATE acquired them, and you'll see ESDI drives that use RLL
and other that use ERLL and still others using MFM. Likewise with SCSI
products and SMD.

Real confusion comes from the fact that people referred to drives as being
RLL or MFM drives, which was, perhaps, a difference, but not in the drives.
There were a few models that performed badly with RLL that performed better
(more reliably) with MFM, but those were the exception rather than the rule.
The drive, actually the head/media combination, determined the flux reversal
density, and the spindle speed determined the resulting bit rate, right? It
seems simple enough. Sadly, there's more ... particularly after the
adoption of ZBR, in which the data rate with respect to time borrowed from
the strategy of CLV recording as used with optical media as opposed to the
previously popular CAV. It's no wonder that folks are confused, but it's
been 10 years or more since MFM fell out of fashion, and, in fact, since
controllers became a drive function rather than a host adapter function.
You'd think folks would have gotten these concepts straightened out. <sigh>

<getting down from soapbox>

That drive I'm looking for has to have at least 8 heads, and it has to have
at least 640 cylinders. In this particular case, until I find my source
code, I can't benefit from more heads or cylinders, nor can I "get by" with
fewer. I have plenty of drives that don't fit this model, but I want one
that will, to help stomp out an immediate fire.

Knocking out an interface that would handle all the modulation schemes, data
rate variations, and wiredo data rates, etc, might not be so easy as it
looks. Yes, one could analyze the format with a combination of various
hardware and firmware techniques, and that way one could continue to use
hardware that would otherwise be unsupportable, but that's the sort of thing
that companies like Adaptec or Mylex spend megabucks to accomplish for just
ONE of the modulation types. The reason that's what's spent is because it
can't really be done for much less. If you want to do that, then have at

I've done lots of this sort of work, yet nobody's offered me a dime to build
an RLL or MFM controller for an ST506-type drive in the last ten years.
It's much easier to replace the drive with one of the more modern ones,
rated, yet not really expected, to have a lifetime of >100K hours, while the
ones of 10 years ago were rated for MTBF's of 5-10 KHours. I can't see how
anybody could justify the investment in technology that will croak in a year
or two. It's easier to rework the firmware and replace the old, 150-lb SMD
with a 150-gram microdrive of 20x the capacity. You can mount that on the
board next to one of the IC's and it will get lost.


----- Original Message -----
From: <>
To: <>
Sent: Tuesday, December 12, 2000 1:13 PM
Subject: Re: need a HDD

> > Date: Tue, 12 Dec 2000 16:33:06 -0800
> > From: Marvin <>
> > To:
> > Subject: Re: need a HDD
> > Reply-to:
> >
> >
> > Richard Erlacher wrote:
> > >
> > > Just in case someone's got one, I'm looking for a hard disk with 640
> > > cylinders x 8 heads to replace a Rodime 204E. If anyone has somthing
> > > will fit this geometry, and it's available, I'd like to be informed.
> >
> > My memory (and docs) are not so good right now, but isn't that basically
> > same configuration as the Seagate 251? If so, I probably have several of
> > them just sitting around *waiting* for an application to pop up :). Let
> > know if you need one.
> Mavin,
> In MFM world, most commonest n of heads and cylinders is usually 6xx
> and 8xx and 4 to 6 heads. Beyond that, that not very common.
> ST251 is 820 (check) x 6 heads (not met) needs 8.
> On this topic,
> Even I was latecomer to MFM I deal with MFM/RLL, exrotic ESDI in
> early '90's.
> While I was thinking, why not have someone design and program a
> adapter to take MFM/RLL as well as ESDI and translate it to SCSI or
> IDE and wrap it up in DSP and all in one microcontroller, interface
> chipset and some ram, rom? This way, helps people to upkeep their
> oddball machine that insists on odd drives. To set up that adapter
> board to report specific cylinders range and heads, N of sectors that
> reports back to that host machine and just "LL format" the scsi or
> IDE hd if needed then HLF it in normal manner as you would do with
> any MFM/RLL, ESDI drives.
> Cheers,
> Wizard
Received on Tue Dec 12 2000 - 20:23:15 GMT

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