14 Nov
2007
14 Nov
'07
8:18 p.m.
I was reading your thread about DDC head-per-track drives.
I had a friend around 1980 that had one of these drives which he had
obtained as surplus from the US Air Force.
It used helium atmosphere around the disks to reduce friction. The disk was
12" in diameter and spun at 33,000 RPM (not a typo), and so it would produce
considerable heat from friction if ordinary air were used, at least that's
what I heard at the time. It was intended as a substitute for magnetic core
(RAM) for mainframes. The disk latency was around 30 microseconds. Because
the driving electronics were a lot slower then, the rated random access
speed was (as I recall) 50 microseconds.
Lee
15 Nov
15 Nov
7:40 a.m.
On Wednesday 14 November 2007 15:18, Lee Inness-Brown wrote:
I was reading your thread about DDC head-per-track
drives.
I had a friend around 1980 that had one of these drives which he had
obtained as surplus from the US Air Force.
It used helium atmosphere around the disks to reduce friction. The disk
was 12" in diameter and spun at 33,000 RPM (not a typo), and so it would
produce considerable heat from friction if ordinary air were used, at least
that's what I heard at the time. It was intended as a substitute for
magnetic core (RAM) for mainframes. The disk latency was around 30
microseconds. Because the driving electronics were a lot slower then, the
rated random access speed was (as I recall) 50 microseconds.
One thing I've been curious about with regard to drives like that -- how many
heads/tracks does it have? How many _can_ such a thing have?
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
7:49 a.m.
On 14 Nov 2007 at 12:18, Lee Inness-Brown wrote:
It used helium atmosphere around the disks to reduce
friction. The disk was
12" in diameter and spun at 33,000 RPM (not a typo), and so it would produce
considerable heat from friction if ordinary air were used, at least that's
what I heard at the time. It was intended as a substitute for magnetic core
(RAM) for mainframes. The disk latency was around 30 microseconds. Because
the driving electronics were a lot slower then, the rated random access
speed was (as I recall) 50 microseconds.
Let's see--33,000 RPM is 550 rev/second or about one rev every 1.8
milliseconds. To get that down to 60 microseconds max (2*30) you'd
have to pack 36 heads per track. That's a lot of heads on a 12"
disk!
Cheers,
Chuck
8:23 a.m.
On Wed, 2007-11-14 at 23:49 -0800, Chuck Guzis wrote:
Let's see--33,000 RPM is 550 rev/second or about
one rev every 1.8
milliseconds. To get that down to 60 microseconds max (2*30) you'd
have to pack 36 heads per track. That's a lot of heads on a 12"
disk!
Even assuming only a 3" band of the disk was available (radius being 6",
a bit around the edge and a bit for the spindle), that's still only 12
tracks per inch. No-one said the heads all had to be in a line, either.
You could mount four head bars with three heads apiece.
Gordon
6:41 p.m.
On 15 Nov 2007 at 8:23, Gordon JC Pearce wrote:
On Wed, 2007-11-14 at 23:49 -0800, Chuck Guzis wrote:
I suppose I'm not being clear here. If latency is 30 microseconds,
then maximum access time is 60 microseconds. The disk completes a
rev in 1.8 milliseconds. To meet the latency requirement, that means
that anything written must be read no later than 60 microseconds
after being written. The only way I know to do this is by employing
multiple heads per *track*, spaced around the track so that any head
can be used to pick up data just written. If your drive has, say, 50
tracks, that implies 36*50 = 1800 heads.
Maybe my math is messed up, or the OP dropped a zero somewhere, but
that's what I get.
Cheers,
Chuck
Let's see--33,000 RPM is 550 rev/second or
about one rev every 1.8
milliseconds. To get that down to 60 microseconds max (2*30) you'd
have to pack 36 heads per track. That's a lot of heads on a 12"
disk!
Even assuming only a 3" band of the disk was available (radius being 6",
a bit around the edge and a bit for the spindle), that's still only 12
tracks per inch. No-one said the heads all had to be in a line, either.
You could mount four head bars with three heads apiece. 
8:32 p.m.
from at fu3.org wrote:
2007/11/14, Lee Inness-Brown <leeib at sequan.com>:
You'd have to have a killer transmission to spin a diesel up to that
speed. 8-)
Peace... Sridhar
It used helium atmosphere around the disks to
reduce friction. The disk was
12" in diameter and spun at 33,000 RPM (not a typo),
Hm, any idea what made it spin? - Diesel engine? Heh.. 
9:15 p.m.
2007/11/15, Sridhar Ayengar <ploopster at gmail.com>:
from at fu3.org wrote:
=]
2007/11/14, Lee Inness-Brown <leeib at
sequan.com>:
You'd have to have a killer transmission to spin a diesel up to that
speed. 8-)
It used helium atmosphere around the disks to
reduce friction. The disk was
12" in diameter and spun at 33,000 RPM (not a typo),
Hm, any idea what made it spin? - Diesel engine? Heh..
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