Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site abnji.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!mhuxv!abnji!jeff From: jeff@abnji.UUCP (jeff) Newsgroups: net.micro Subject: confused about disk drive specifications Message-ID: <735@abnji.UUCP> Date: Wed, 10-Jul-85 15:43:28 EDT Article-I.D.: abnji.735 Posted: Wed Jul 10 15:43:28 1985 Date-Received: Fri, 12-Jul-85 01:44:53 EDT Lines: 140 [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 | +----------------------------------------+