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

[Franc Zabkar]

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

 

[calypso]

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...

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

--
U kaficu se svakih 15 minuta glup nogometasog gnjecija.
By runf

Damir Lukic, calypso@_MAKNIOVO_fly.srk.fer.hr
http://inovator.blog.hr
http://calypso-innovations.blogspot.com/

 

[mscotgrove]

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

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

--
U kaficu se svakih 15 minuta glup nogometasog gnjecija.
 By runf

                                  DamirLukic, calypso@_MAKNIOVO_fly.srk.fer.hr
                                 http://inovator.blog.hr
                                 http://calypso-innovations.blogspot.com/

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


Michael
www.cnwrecovery.com

 

[Arno]

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

 

[Guest]

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.

 

[Franc Zabkar]

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

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

 

[Franc Zabkar]

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

 

[Franc Zabkar]

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

 

[Arno]

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

 

[Franc Zabkar]

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

 

[Oscar]

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.

 

[Eric Gisin]

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

 

[John Turco]

Franc Zabkar wrote:

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 <[email protected]>

 

[Franc Zabkar]

Franc Zabkar wrote:



<edited>

Hello, Frank:

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

Actually my question was a rhetorical one.

It appears that Spinrite may have no screen shots of modern drives,
which begs the question, just how useful is it?

- Franc Zabkar

 

[Arno]

Actually my question was a rhetorical one.

It appears that Spinrite may have no screen shots of modern drives,
which begs the question, just how useful is it?

Hehe, not at all?

Basically it can do the same as the disk istelf can, when you run
a long SMART selftest. It used to be diffwerent, when disks
were dumb, but not anymore.

Arno

 

[John Turco]

Actually my question was a rhetorical one.

Hello, Frank:

Yes, indeed...I'd suspected as much. I believe that sharing such a nugget of HDD history makes

It appears that Spinrite may have no screen shots of modern drives, which begs the question,
just how useful is it?

- Franc Zabkar

Not very, I'd imagine. :-J


Cordially,
John Turco <[email protected]>

 

[Barry OGrady]

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.


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.

I picked up an ancient PC at an auction. It had a full height 5.25" 10MB
hard drive and full height 5.25" floppy drive. I was surprised to see that
the floppy drive used 80 tracks double sided and variable sectors per
track to reliably store 1.1MB on a standard double density disk.
The motor speed was varied every 10 tracks. The hard disk had MSDOS
1.? on it which does not allow for subdirectories, so it was configured
with the hard drive as drives A, C, D and E, with the floppy as B.

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

- Franc Zabkar

Barry
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