Path: utzoo!attcan!uunet!steinmetz!davidsen
From: davidsen@steinmetz.ge.com (William E. Davidsen Jr)
Newsgroups: comp.sys.ibm.pc
Subject: Re: RLL controller
Message-ID: <12650@steinmetz.ge.com>
Date: 22 Nov 88 18:49:54 GMT
References: <299@deimos.cis.ksu.edu> <400@mccc.UUCP> <1038@pilchuck.Data-IO.COM> <407@mccc.UUCP>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: General Electric CRD, Schenectady, NY
Lines: 53

The reliability of RLL (or any self clocking recording) is influenced by
the accuracy of timing of the disk and the controller. The window is the
sum of the timing slop.

Like any product, disks and controllers have tolerances. If they are set
too tight the product cost gets very high, and if they're too low the
product is unreliable. Any non-RLL disk may still have timing margins
tight enough to run RLL. The RLL models (like ST238 vs ST225) are
testing to tighter requirements than the MFM models.

Disk controllers have a range, too, and any given controller may work
at the limit of the tolerance or half that. This all means that some MFM
disks will work for RLL, and some RLL controllers will require close
timing from the disk.

Now think about the stories you hear about good and bad products; the
person who has three Seagates which don't work well and then gets a
Miniscribe which works "for two years." Does he have a sloppy controller
which requires a really tight disk timing? Would a third Seagate have
been just as good?

Did the lady who was saying that Adaptek controllers are bad and that
she went to WD and had no trouble have bad controllers? Or was her disk
sloppy, and did she get a really good controller the last time around.

What you have is always "the best," since you would have something else
if it wasn't. I've been listening to the ST225 wars, with the people who
bought early ones say "it's been working for years, and I drag it up the
stairs every night with a rope," while others say "I had two old ones
and they were noisy, but the new one is okay." If the old one was bad
the first guy would have a new one, etc.

For the record, I have used non-RLL disks without problems, but I look
at the timing info on the data sheet when I can. I have been able to
find MFM disks which were close enough to pass for RLL. I have one disk
which has two tracks which "drift" as RLL and worked for months with
MFM. I have used Adaptek and Perstor and like them both. The old ST225
was not recommended for RLL use, Perstore says the new ones will work,
so I guess something other than the labels have changed.

Hope this helps clarify why people are having such disagreements about
disks and RLL. If they use a small number, or even a large number from
one batch, they may get results which don't match yours. Even the
shipper may be at fault, or where you are on the delivery route (the
package at the end of the route may get less love from a tired driver).
It's good that people report their experience, but bad when they say "I
had a bad one, they're all crap," or "mine has run for three years,
they're really reliable."

-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me