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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. |
#2
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3753 RPM for old PC/AT 20MB MFM hard drive
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 |
#3
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3753 RPM for old PC/AT 20MB MFM hard drive
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. |
#4
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3753 RPM for old PC/AT 20MB MFM hard drive
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. |
#5
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3753 RPM for old PC/AT 20MB MFM hard drive
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. |
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3753 RPM for old PC/AT 20MB MFM hard drive
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3753 RPM for old PC/AT 20MB MFM hard drive
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#8
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3753 RPM for old PC/AT 20MB MFM hard drive
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. |
#9
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3753 RPM for old PC/AT 20MB MFM hard drive
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. |
#10
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3753 RPM for old PC/AT 20MB MFM hard drive
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 |
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