Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!vsi1!altos86!dtynan
From: dtynan@altos86.Altos.COM (Dermot Tynan)
Newsgroups: comp.os.minix
Subject: Re: Device drivers damaging HW
Summary: The *correct* answer...
Message-ID: <12390@altos86.Altos.COM>
Date: 2 Aug 89 04:27:58 GMT
References: <13679@ea.ecn.purdue.edu> <4697@omepd.UUCP> <8804@attctc.Dallas.TX.US>
Organization: Altos Computer Systems, San Jose, CA
Lines: 68

In article <8804@attctc.Dallas.TX.US>, chasm@attctc.Dallas.TX.US (Charles Marslett) writes:
> In article <4697@omepd.UUCP>, pcm@iwarpj.intel.com (Phil C. Miller) writes:
> > In article <13679@ea.ecn.purdue.edu> jel@ecn.purdue.edu (Jim Lumpp) writes:
> > > ... How susceptible
> > >is the IBM hardware to such damage?
> 
>  [Much discussion about various facets of video display design, etc.]


OK, here is the technical answer to your questions...
	1.  Display adaptors.
	The video beam on your display scans across from left to right, and
	from top to bottom.  It uses magnetic fields to divert the beam.
	It takes 63.5us (in the US) to go from one side to the other.  This
	is a frequency of 15.750kHz.  In order to produce the high voltage
	for the display, a transformer is needed.  This transformer has
	to be switched at a high frequency, so they use the horizontal
	sync frequency.  In short, the operation is similar to that of a
	car ignition coil.  However, if the sync frequency changes too
	far from the norm, the transformer will begin to burn up, due to
	the change in inductance (and as a result, current).  Before it
	does this, one will usually hear a high-pitched whistle from the
	display.  This is a definite indicator that things are not well.
	It is possible on most (if not all) graphics adaptors to change
	the sync frequency.  Playing around with these values can result
	in damage to your monitor.

	2.  Disk drives.
	Standard floppy-disk drives have track-zero override on the
	stepper motor.  This means, that no more stepping pulses in
	the direction of the edge will be accepted, when the head is
	at track zero (outside track).  This is so that you can home
	the head, by sending 255 step requests to the drive.  The
	apple ][ does this, but they removed the track-zero sensor,
	that is why you hear those drives make that grinding noise
	when they are reset.  However, even on the PC, there is no
	protection in the other direction.  This means that you can
	issue as many step requests toward the hub, and the drive
	will attempt to move the head, resulting in the stepping
	mechanism resetting itself.  This also results in a grinding
	noise (one I might add, I've heard from Minix on occasion).
	This is not good for the mechanism.  Commercial drives are
	not very robust, and tend to wear if this practice is repeated.
	Usually it will manifest itself in a fashion untested by most
	repair shops.  The alignment will no longer be linear (ie,
	aligned on the centre tracks, but - on the inner, and + on the
	outer, or, of course, vice-versa).
	A second destructive mechanism for drives, is head chatter.
	This is the repeated loading and unloading of the heads.  Again,
	Minix is a big offender in this regard.  As a general rule,
	the software should wait five seconds after the last access,
	before unloading the heads.  Double-sided drives are the
	worst.  Eight-inch drives, if you have any are extremely
	critical, and can be damaged by incorrectly inserting a disk.
	That of course, is a different problem..

	3.  Line printers.
	This isn't really relevant, so I'll keep it short.  It is also
	possible to do serious damage to your line printer (and I *mean*
	line printer, not Epson toys), if you repeatedly print the
	same line.  It can also cause serious damage to your floor, when
	the printer falls over :-(
						- Der
-- 
	dtynan@altos86.Altos.COM		(408) 946-6700 x4237
	Dermot Tynan,  Altos Computer Systems,  San Jose, CA   95134

    "Far and few, far and few, are the lands where the Jumblies live..."