Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!crdgw1!sixhub!davidsen
From: davidsen@sixhub.UUCP (Wm E. Davidsen Jr)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MFM as an RLL drive?
Message-ID: <254@sixhub.UUCP>
Date: 27 Dec 89 00:38:25 GMT
References: <887@crash.cts.com> <28373@amdcad.AMD.COM> <44809@bu-cs.BU.EDU> <423@hitech.ht.oz>
Reply-To: davidsen@sixhub.UUCP (bill davidsen)
Organization: *IX Public Access UNIX, Schenectady NY
Lines: 54

In article <423@hitech.ht.oz> clyde@hitech.ht.oz (Clyde Smith-Stubbs) writes:
| ngeow@bu-cs.BU.EDU (Yee Ngeow) writes:
| 
| >  Please note that running an MFM drive as RLL DEFINITELY CAN HURT
| >it physically. Running an MFM at RLL push the drive electronics at
| >50% higher clock rate then an MFM. As with any electronic components, this
| 
| Would someone who knows what they are talking about please stand up
| and clear this up: to my knowledge the difference between MFM and RLL is NOT
| that the clock rate is different, but that RLL has less redundant information,

  There are some really good articles on this, posted by Pete Holzman
at Octopus. I have them on my work machine, but I'm on vacation. The
gist is that RLL reduces the number of bits by using timing info to
*reduce* the flux changes used to represent data. What we call MFM is
technically also RLL, by the way, something like RLL 1,3, while what is
usually called RLL is 2,7. I could explain this, but why bother when
there are articles which do it better.

  When running an RLL controller the rate of flux changes is actually
less, although the information data rate from the bus to the controller
*is* higher.

| i.e. to record one MFM bit takes two transitions while to record one
| RLL bit takes only 1 point something transitions.

  That's the difference between FM and MFM, but you have the right idea.
RLL works the disk smarter, not harder. If the drive media, drive speed
flutter, or electronics are not up to RLL it won't work, but nothing bad
will happen to the drive (just the data). Typically the first 24 hours
show problems if you are going to get them, although something just on
the edge might tend to fail in the summer or winter (air conditioning or
electric heat line noise) if your luck is very bad.


| Incidentally I have formatted a Micropolis 1335 (MFM) as RLL and it came
| up ERROR-FREE! When formatted as MFM it has bad sectors. I still entered
| the factory bad tracks manually to be safe.

  I have noticed this, too. When I went from Adaptek to WD RLL, my ST251
showed no bad tracks. That was about six months ago. The other day I
stated getting "can't read" messages, and I checked the whole disk again
for bad tracks. Like to rabbit commercial it's "still going." The
message came from a program trying to do something fancy with the disk
and reading outside a partition.

  I'm running an ST4096, too, with no problems, although it had a few
bad tracks and is way over the rated MTBF. I keep current backups on ALL
my systems, so I don't treat RLL as special.
-- 
	bill davidsen - sysop *IX BBS and Public Access UNIX
davidsen@sixhub.uucp		...!uunet!crdgw1!sixhub!davidsen

"Getting old is bad, but it beats the hell out of the alternative" -anon