Ressurecting FDDs (was Original IBM PC (was Re: Prices to

From: Lawrence Walker <lwalker_at_mail.interlog.com>
Date: Tue Jun 2 10:52:54 1998

On 2 Jun 98 at 1:45, Tony Duell wrote:

> > What exactly does alignment entail? What part is getting misaligned?
>
> For a floppy driver, there are essentially 4 adjustments that you have to
> make to do a full alignment :
>
> 1) Spindle speed (not on synchronous-motor drives). This one is obvious
>
> 2) Head radial position. This involves adjusting the head position
> radially with respect to the spindle so that it's positioned over the
> centre of a data track, and not off to one side. This is the one that
> most people mean by 'alignment'. You use a special disk recorded with
> ofset tracks, look at the signal from the head on a 'scope and move the
> heard (or more correctly the stepper motor) so that the 2 offset tracks
> are the same amplitude on the 'scope
>
> 3) Track 0. You've now got the heads over a track. And you know which
> track it is, since the alignement disk only has said pattern on selected
> tracks. Now step the head out the appropriate number of tracks and adjust
> the track 0 sensor (switch/optoswitch) so that it operates at the right place
>
> 4) Index timing. This one is not that important, actually, on most
> controllers. One track on the alignment disk had a burst of data
> positioned accurately wrt the index hole. What you do is read at that
> track, look at the index pulse and the data burst on the 'scope and move
> the index sensor until the delay between them is right.
>
> In general only (2) and (3) are that important with modern controllers.
> All of them are important on hard-sectored controllers, though.
>
> > Does this happen to any other type of drive (CD-ROM, HDD,etc.)
>
> OK... Modern hard disks don't need alignment - they're servo-tracked, and
> the head uses information on the disk platters - the so-called servo
> bursts - as a position reference. Older demountable hard disks in general
> either used one surface of a multi-platter stack for this servo
> information (when you have to align the heads wrt each other), or had a
> stepping actuator (not, in general as stepper motor, but an actuator that
> took its reference from something in the drive and not on the disk), when
> you have to align the heads wrt the spindle. You do these like a floppy
> drive, with an alignment pack and a 'scope.
>
> Some older winchesters have a stepper motor. In general there's not a lot
> you can do with these other than to low-level format them and keep on
> using them. Of course you lose data if you do that.
>
> CD-ROMs don't need radial alignment as such - they're servo-positioned in
> general. The average CD-ROM drive has 3 or 4 servo loops - focus, spindle
> speed, radial position (or coarse and fine radial postion). There are
> electronic adjustments for these sometimes, and maybe also a laser power
> control. Don't fiddle without a service manual!. You set these up like an
> audio CD player.
>
> Tape drives also have alignment adjustments. The old 9-track drives (this
> is classiccmp, after all!) have motor speed, tape tension, head skew
> (basically angle of the head), sometimes head tilt, tape sensor, etc
> adjustments. Even cartridge tape drives have tape speed, head height and
> maybe head azimuth adjustments.
>
> I'll leave paper tape units, card readers, printers, teletypes and
> monitors until another time (read : until someone asks about them).
> Suffice it to say they all need alignment sometimes.
>
> -tony
>
 Ahh, I knew you had a fdd "how-to" simmering in there Tony. " ^ ))
This one should definitely go in the FAQ.

ciao larry
lwalker_at_interlog.com
Received on Tue Jun 02 1998 - 10:52:54 BST