jeff@abnji.UUCP (jeff) (07/10/85)
[whurr! clank! clikety-click!]
I am confused about floppy disk drive specifications.
I see that the major choices are:
. single sided single/double density 48 TPI
. double sided single/double density 48 TPI
. double sided single/double density 96 TPI
full or half height.
John J. Meshna Jr. (a surplus catalog) has the Mitsubishi
no. 4853 for $175. It is double sided, double density, 80 tracks
per side. Is this the same as a DSDD 96 TPI drive?
The way I understand the specs:
single sided - read/write heads on one side only
You cannot flip over the disk to use the other side unless
the sector hole is in the center, or the sector hole is not
used (as in Apple computers).
double sided - read/write heads on both sides
density - depends on the external controller, thus any drive
that can handle double density can be used single density.
Does quad-density mean another doubling of the density per track
by the controller, as long as the drive can handle it?
I think density refers to the number of bits per track.
How do they specify how many tracks the head moves (and is this
necessarily all the tracks on the diskette?)? I think the TPI rating
and the number of tracks per side are related as follows:
2.25" outer track radius
- 1.4373" inner track radius
------
.8125" head movement edge to edge
so 48TPI means (48 tracks/inch) (.8125 inches) + 1 = 40 tracks
(a track is a discrete head position), thus the 80 tracks
means 96TPI.
If a drive claims 160 tracks - is that 80 tracks by 2 sides
at 96 TPI, or 160 tracks per side at 192 TPI?
Is this a reasonable approximation of reality?
Let us try to read the following specs:
mfg. model tracks no.heads
==== ===== ====== ========
a)Shugart 400 35 1
b)Shugart 400 35 1
c)BASF 6108 80 (both sides) 2
d)Pertec FD 250 70 (80 optional)2
e)MPI B92 160 (both sides)2
f)CDC 9409T 160 (both sides)2
g)Mitsubishi 4853 160 (both sides)2
Drives a and b could be 48 TPI with the head not traveling all the
way to the hub, or (35 tracks)/(.8125 inches) = 39.4 TPI.
Which is it?
Drive c is 80 tracks total, 40 per side thus 48TPI.
Drive d - Does the stepper mechanism have two modes,
stepping .8125/(70-1) inches or .8125/(80-1) inches per track?
With 70 tracks, the TPI is 1/(space between tracks) = 85 TPI.
With 80 tracks, it is 1/(.8125/(80-1)) = 97 TPI.
What did I calculate wrong - the head traveling distance?
How - by changing the lead screw?
Drive e, f, and g are 160 tracks total, 80 per side thus 96 TPI.
In conclusion:
I think it is too easy to confuse tracks total with tracks per side
(total tracks = tracks per side times number of sides).
The TPI rating says the distance between tracks, but not how many the
heads will reach. It should not be confused with density although
a 96 TPI disk will hold twice the data as a 48 TPI disk. I wonder
if some of the 'quad density' drives are really double density
at 96TPI as compared to single density at 48 TPI.
The number of tracks the head can position itself is almost standard
(80 for 96 TPI, 40 for 48 TPI) but other possibilities exist.
Does this mean that the tracks for a 35 track and 40 track drive
won't align due to unequal spacing (thus different TPI ratings)?
Density is the number of bits per track, determined by the
controller. The drive dictates how fast the data can be
read/written thus limiting the density which is otherwise
determined by an external controller.
One unit of measure is the number of bits per inch along
the track. Most controllers write the same number of bits on the
outer track as the inner track, thus the number of bits per track
is constant.
Since the circumference of the outer tracks is larger than the inner tracks
and assuming the density in bits per inch along the track being
constant, some controllers
therefore put more bits on the outer tracks so the density of
bits per track can vary from the outside to the inner track.
Height has no effect on the drive's operation except that
some full height drives can lift the heads when the disk is
not in use and even shut off the motor. The half height drives
apparently don't have the room to do this and it makes for a
less expensive head mechanism at the expense of additional
disk wear.
Speed of rotation is constant, isn't it? That would affect
the density. Are the speeds (in RPM) standard?
I suppose a disk written at one speed can be read at another speed if
the format is self-clocking and the controller is adaptive enough.
Is wow and flutter a consideration, or does the controller compensate
for speed variations. Even so, does this affect the reliability of
the drive (a more stable motor meaning less read errors?)?
Density of the medium - the finer the magnetic particles in the coating
the more magnetic fluxes per inch can be stored. Is the density
uniform in all directions or only in a particular orientation
(such as along the tape's or disk's tracks)? Tapes are rated
in FCI (flux changes per inch, which determines BPI and TPI).
Why not disks?
There are several mechanisms for moving the heads. One I have seen
moves the heads via a metal band directly to the motor pulley.
It is quite fast and quiet. Others use a lead screw, which
needs occasional cleaning and lubrication. Do any use linear
motors, or is that an unwarranted expense for a floppy drive?
This might appear as a spec on head seek time.
My Teac drives have a 'button type' head. I have seen DSDD
drives that have small heads on a leaf spring. Are there others?
Unless you get the manufacturer's specs, this is not usually
part of the drive specification.
My fingers grow weary, so I conclude with a request for someone
knowledgable to offer a dissertation on disk drive specifications.
Thank you for reading all this, and in advance (for whatever).
+----------------------------------------+
| Jeff 'I am sooooooooo confused' Skot |
| at the bewildering ATT IS Somerset |
| {ihnp4 | mcnc | cbosgb} abnji ! jeff |
+----------------------------------------+hes@ecsvax.UUCP (Henry Schaffer) (07/11/85)
I have always heard single vx. double density to refer to bits per track, while quad density was loosly used to mean 98 tpi or any other method of giving twice the amount of storage available from double density (double density on a double sided 5 1/4" is approx 316k - 370k, and so quad means >=600k.) --henry schaffer
sienkiew@UDEL-LOUIE.ARPA (07/12/85)
Well, this isn't a thesis, but it may help: A 35 track drive has the tracks in the same place as a 40 track drive does. You can read disks from one on the other if you have to. (Of course, the 35 track drive can't get the last few tracks from a 40 track disk...) In practice, I have used 36 tracks on 35 track drives and 41 tracks on 40 track drives with no problems. It seems to me that a lot of 40 track drives only ever get to the first 35 tracks. I've hacked Apple DOS 3.3 to use 41 tracks with no problem, but I had to do it myself--it didn't come with the drive. From what I've read about 96 TPI drives, they have twice as many tracks in the same space as 48 tpi drives. Thus, if you only read every other track, you can read a 48 tpi disk. You can't write one because the signal will be too weak for a 48 tpi disk to pick up. [has anybody ever heard of a 96 tpi disk that adjusts the head current to write 48 tpi disks?] Disks have to be approached differently from tapes, because a tape can be made to move the media at a fixed rate. A disk changes the rate each time you go to a different track. Your controller isn't really writing Bits Per Inch--it's writing Bits Per Second. I'm sure some creative engineering could get around that, but I'm not aware of any. (I have heard rumors that some disk systems vary the timing based on the track number so they can get more data on the outside tracks--can anybody verify this?)
jjg@ccice5.UUCP (John Grana) (07/16/85)
> > Disks have to be approached differently from tapes, because a tape can be made > to move the media at a fixed rate. A disk changes the rate each time you go to > a different track. Your controller > isn't really writing Bits Per Inch--it's writing Bits Per Second. I'm sure > some creative engineering could get around that, but I'm not aware of any. > (I have heard rumors that some disk systems vary the timing based on the track > number so they can get more data on the outside tracks--can anybody verify > this?) The IWM (Integrated Woz Machine) in the Mac uses the above to get more sectors on the outer tracks... !rochester!ccice5!jjg
rlk@wlcrjs.UUCP (Richard L. Klappal) (07/17/85)
[] I believe the Victor 9000 had (has?) a variable speed drive that put more sectors (or maybe more bytes/sector) on the outer tracks. If anyone from FORD is on the net, thay might confirm, since they started to standardize on the Victor before their (Victor's) finacial problems. Richard Klappal UUCP: ..!ihnp4!wlcrjs!uklpl!rlk | "Money is truthful. If a man MCIMail: rklappal | speaks of his honor, make him Compuserve: 74106,1021 | pay cash." USPS: 1 S 299 Danby Street | Villa Park IL 60181 | Lazarus Long TEL: (312) 620-4988 | (aka R. Heinlein) -------------------------------------------------------------------------
magik@wlcrjs.UUCP (Ben Liberman) (07/19/85)
>> (I have heard rumors that some disk systems vary the timing based on the track >> number so they can get more data on the outside tracks--can anybody verify >> this?) > > The IWM (Integrated Woz Machine) in the Mac uses the above to get more > sectors on the outer tracks... > If I remember correctly, the Victor 9000 used 5 different speeds zones for the same reason. -- ----------------------------------------- Ben Liberman {ihnp4|ihldt}!wlcrjs!magik
ral@pyuxqq.UUCP (R A Levenberg) (07/23/85)
Re: I have heard rumors that some disk systems vary the timing based on the track number so they can get more data on the outside tracks--can anybody verify this? I remember when HP first brought out the HP 150, it used 3.5" floppies that handled only 256K per disk. Then along came the Macintosh, which used the same 3.5" disks, but by varying the speed of the drive depending on the seek address, the Mac was able to get 400K on each floppy. ======================================================================== Ron Levenberg Bell Communications Research 3 Corporate Place Room 2C-315 Piscataway, NJ 08854 (201) 981-6178 ..!allegra!pyuxqq!ral
ral@pyuxqq.UUCP (R A Levenberg) (07/29/85)
The following was MAILed to me, but I was not the person who
posted the query, so I've attached the article for posting.
Ron Levenberg
Bell Communications Research
3 Corporate Place
Room 2C-315
Piscataway, NJ 08854
(201) 981-6178
..!{ihnp4,allegra}!pyuxqq!ral
========================================================================
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Subject: Re: confused about disk drive specifications
References: <11527@brl-tgr.ARPA> <868@ccice5.UUCP> <752@wlcrjs.UUCP>, <741@pyuxqq.UUCP>
The Victor 9000 and the newer Victor models - Vicki & Vi vary the drive speed.
There are eight or nine speeds (slower speeds when you read from or write to
the outside tracks. This gives the Victor 5.25" drives a capacity of 1.2
megabytes per each double-sided drive.