3753 RPM for old PC/AT 20MB MFM hard drive

I was looking at the specs for the 20MB MFM hard drive in the original
IBM PC/AT. The Tech Manual states that the rotational speed was 3573
RPM +/- 0.5%. The motor is a 3-phase DC type. I'm wondering why 3573
RPM and not 3600 RPM?

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote:
I was looking at the specs for the 20MB MFM hard drive in the original
IBM PC/AT. The Tech Manual states that the rotational speed was 3573
RPM +/- 0.5%. The motor is a 3-phase DC type. I'm wondering why 3573
RPM and not 3600 RPM?

3600rpm is 60 rps (rounds per second). 60Hz is also the
US AC, AFAIK (Europe uses 50Hz). Maybe they just wanted to avoid
being exactly on the AC and made it 0.75% lower so as to be
reliably at least 0.25% lower than the AC. Just a thought, I have
no idea whether that was the reason.

Arno0

On Sun, 26 Apr 2009 03:20:51 -0700 (PDT), "
put finger to keyboard and composed:

The disk platters are fixed (ie not removable), and so the spin rate
is not very critical - as long as it is always the same.

I don't think that the speed needs to be always the same. I would
think that there would be a fairly wide tolerance since the clock is
extracted from the MFM data. The early MFM drives also provided an
Index pulse to the controller.

For a floppy
disk, as the media is removable a standard spin rate is essential.

A 1.2MB 360RPM 5.25" floppy drive can read a diskette written in a
360KB 300RPM 5.25" drive. In order to do this, the FD controller's
data separator has to be configured for a transfer rate of 300 Kbits/s
instead of the original 250 Kbits/s.

http://www.pcguide.com/ref/fdd/formatSummary-c.html

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Sorry for the typo in the subject line. It should say 3573, not 3753.

On Sun, 26 Apr 2009 00:49:36 +0000 (UTC),
lid put finger to keyboard and composed:

Franc Zabkar kenjka:
I was looking at the specs for the 20MB MFM hard drive in the original
IBM PC/AT. The Tech Manual states that the rotational speed was 3573
RPM +/- 0.5%. The motor is a 3-phase DC type. I'm wondering why 3573
RPM and not 3600 RPM?

I believe that 3600rpm is just rotational speed they wanted to achieve, but
it was not always possible...

I would think that the speed would have been locked to a quartz
crystal or a piezoelectric ceramic resonator.

According to ...

http://en.wikipedia.org/wiki/Ceramic_resonator

.... "quartz has a 0.001% frequency tolerance, while PZT has a 0.5%
tolerance".

Modern hard drives use quartz crystals, and floppy drives appear to
use ceramic resonators, but maybe the early MFM drives also used
resonators for speed control.

The fact that 3573 RPM (= 59.55 Hz) is a precise figure suggests that
it was in fact the target RPM. Maybe it was specifically chosen to
avoid US mains frequency interference issues, as Arno has suggested???

Try with Spinrite to see how the speed changes even on today's drives...

I haven't encountered Spinrite since the old MFM days. Does the speed
vary on either side of the target speed, or is it consistently low?

BTW, I notice that the screen shots on Steve Gibson's web site
(http://www.grc.com/srscreens.htm) all seem to refer to very old
drives.

This one appears to be for a 20MB MFM drive:
http://www.grc.com/image/srGSD.gif

When did Conner Peripherals go out of business?
http://www.grc.com/image/srSAM.gif

Which 210MB 3609 RPM drive is this?
http://www.grc.com/image/srcharacteristics.gif

When was 2.8 Mbytes/sec a state-of-the-art burst transfer rate?
http://www.grc.com/image/srbench.gif

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

On 26 Apr 2009 17:42:22 GMT, Arno put finger to
keyboard and composed:

Franc Zabkar wrote:
I was looking at the specs for the 20MB MFM hard drive in the original
IBM PC/AT. The Tech Manual states that the rotational speed was 3573
RPM +/- 0.5%. The motor is a 3-phase DC type. I'm wondering why 3573
RPM and not 3600 RPM?

3600rpm is 60 rps (rounds per second). 60Hz is also the
US AC, AFAIK (Europe uses 50Hz). Maybe they just wanted to avoid
being exactly on the AC and made it 0.75% lower so as to be
reliably at least 0.25% lower than the AC. Just a thought, I have
no idea whether that was the reason.

That occurred to me, too. Perhaps the designer wanted to avoid an
interference issue???

I notice that Google finds several references to 3573 RPM for high HP
AC induction motors. These motors have a "slip". However, the concept
of slip has no relevance to DC motors.

FWIW, I notice that several of the following NEC HDDs rotate at odd
speeds.

10MB, 3600 RPM:
http://support.necam.com/oem/HDD/D5124.asp

20MB, 3564 RPM:
http://support.necam.com/oem/HDD/d5126h.asp

105MB, 3456 RPM:
http://support.necam.com/oem/HDD/D3756.asp

170MB, 3573 RPM:
http://support.necam.com/oem/HDD/D5652.asp

345MB, 4090 RPM:
http://support.necam.com/oem/HDD/D3713.asp

730MB, 3493 RPM:
http://support.necam.com/oem/HDD/D3825.asp

1.08GB, 4500 RPM:
http://support.necam.com/oem/HDD/D3845.asp

2.5GB, 5200 RPM:
http://support.necam.com/oem/HDD/DSE2550A.asp

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

wrote:
It could be that this thing would not do a
1:1 interleave at 3600 but could at 3570 RPM.
Then again, it could just be no reason other
than, that's the RPM it ran.

So maybe designed for 3600rpm, but that just was a bit too fast
for, say, 5% of the drives and with 3570 that dropped to, say,
1%. Makes sense to me.

Arno

On Sun, 26 Apr 2009 18:02:19 GMT, () put finger to
keyboard and composed:

It could be that this thing would not do a
1:1 interleave at 3600 but could at 3570 RPM.
Then again, it could just be no reason other
than, that's the RPM it ran.

These drives used an ST506 interface, ie they did not have an
integrated controller. AIUI, the interleave factor would have been
dependent on controller speed, not the HD.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Arno wrote:
Franc Zabkar wrote:
I was looking at the specs for the 20MB MFM hard drive in the
original IBM PC/AT. The Tech Manual states that the rotational speed
was 3573 RPM +/- 0.5%. The motor is a 3-phase DC type. I'm wondering
why 3573 RPM and not 3600 RPM?

3600rpm is 60 rps (rounds per second).

Revolutions per second, actually.

60Hz is also the US AC, AFAIK (Europe uses 50Hz).

Yes.

Maybe they just wanted to avoid being exactly
on the AC and made it 0.75% lower so as to
be reliably at least 0.25% lower than the AC.

Doesnt explain why all the other drives of that era didnt.

Just a thought,

Steaming turd, actually.

I have no idea whether that was the reason.

Thats obvious.

That's 35.5 rev/s. If you divide a 3.9Mhz clock by 65536 you get 1/35.5.

"Franc Zabkar" wrote in message
...
I was looking at the specs for the 20MB MFM hard drive in the original
IBM PC/AT. The Tech Manual states that the rotational speed was 3573
RPM +/- 0.5%. The motor is a 3-phase DC type. I'm wondering why 3573
RPM and not 3600 RPM?

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote:

heavily edited, for brevity

When did Conner Peripherals go out of business?
http://www.grc.com/image/srSAM.gif

edited

Hello, Frank:

Conner Peripherals was bought out, by Seagate, and merged with the latter company, in 1996.


Cordially,
John Turco