Questions about RLL controllers

Tue Feb 14 10:24:55 AEST 1989

I asked "Can you use a RLL controller on MFM drives?"  I was asked to post
the responses I got to this question, so here they are.

Overall, the picture is mixed; I haven't come out with any clear conclusion.
Some people report extra disk space, faster access and no problems.  Others
tell horror stories.  It's probably not a good idea for the faint-of-heart or
those who can't afford to blow the extra bucks on a wasted RLL controller,
assuming it doesn't work.

Rich Skrenta
-----------
From: Chip Rosenthal <chip at vector.uucp>

In article <3700022 at eecs.nwu.edu> you write:
>Are there any dangers associated with using an RLL controller on drives
>only certified for MFM?

I classify this question in the same catagory as:

    - Can I run an 80286-8 at 10MHz.
    - Can I use 21256-120 RAMs at 100ns.

RLL encoding is "denser", thus you get 26 (512-byte) sectors on each disk
track rather than 17.  Can the surface of your disk handle the higher
density?  At nominal conditions, probably.  At the extremes, all bets
are off.

A lot of people do run MFM-rated disks with RLL controllers.  And a lot
of people do stuff 8MHz CPU's in 10MHz systems.

My paranoia about doing this is because I used to be a test engineer, and
I wrote test programs which do the speed gradings for CPU's.  Think about
it...a manufacturer loses money when a piece of hardware is sold as the
lower performance grade.  If it could, it would rather classify the thing
as RLL or 10MHz or 100ns and charge the increased price.

>Could the controller damage the drive in any way

No.  The risk is of lost data.

>I noticed there are some nasty warnings in the Seagate manual
>about voiding your warranty if you use the wrong controller with a drive.

1.  They don't want to be responsible for lost data.
2.  They want you to buy the more expensive (RLL) disk.

>Also, where does the extra space come from?

Writing more sectors in the same amount of space.

>Do RLL controllers increase
>the sectors per track or the number of tracks?

Sectors per track.

>How would I get Xenix to recognize the new space?

Through the "dparam" program, with which you define all of the
characteristics of the drive.

>Any advice would be appreciated

Running a MFM disk on an RLL controller will probably work.  The question
is, do you want to take the gamble.
---
Chip Rosenthal     chip at vector.UUCP    |      Choke me in the shallow water
Dallas Semiconductor   214-450-5337    |         before I get too deep.
--------
From: "M.R.Murphy" <mrm at sceard.uucp>

[...]

An Adaptec 2372 didn't work with ST251, ST251-1, or ST4096.
Many errors on drives that worked fine MFM. They worked again fine MFM
when I switched controllers again.  I found out the hard way.
Would you summarize to the net?
Regards,
Mike
---
Mike Murphy  Sceard Systems, Inc.  544 South Pacific St. San Marcos, CA  92069
mrm at sceard.UUCP       {hp-sdd,nosc,ucsd}!sceard!mrm            +1 619 471 0655
------------
From: tony%ajfcal%xenlink%calgary%uw-beaver%rochester%ames.uucp%mailrus.cc.umich.edu at cs.umn.edu

The Segate 4096 DOES work correctly in RLL mode on Xenix.  I think
the Miniscribe might also.

Almost any drive using "plated media" disks, (e.g. Segate 4909, all
Priam's etc.) can be used in RLL mode. These have a flux density rating
of at least 10,000 (i.e. coercivity, i.e. a measure of the energy needed
to magnetize). In general, the older red oxide disks cannot be used for
RLL - in part because of the low coercivity).

(Note.  An issue of Byte just came by mail with an article on RLL.  This
might give a better description than mine.  But i have not have time to
read it.)
		[ I went out and got this issue of Byte.  The article is
		  very informative, it tells all about RLL.  However, it
		  doesn't say much about using MFM drives with RLL
		  encoding    -- Rich ]

The "certification" process used by the manufacturers is designed to
determine the number of defective tracks on the disk.  If the
disk is to be used for RLL, then the certification is done with an RLL
tester;  if for MFM, it is done with an MFM tester.  In general,
the manufacturers select the "best" disks for RLL certification - and
therefore command a higher purchase price.

RLL does give noticable increase in performance.  You are now recording
26 sectors per track instead of 17.  In one disk revolution, you can
now (at least theoretically) pick up more information in one revolution.

On the whole, I notice an improvement in speed as well as storage capacity.

I currently have two 4096 drives and two Priam ID60's.  All of these
were "certified" for MFM, however work verrrry fine in RLL. 

To use your segate 4096, the following should be done:

1.	format the disk in RLL format, using a formatter such as
	SPEEDSTOR - setting the "number of sectors per track" to 26.

2.	run an overnite media check on the 4096 to pick up all possible
	bad tracks.  Remember that the drive was certified for MFM.
	If it is used for RLL, a couple of extra bad tracks may be found
	in the testing (None of my drives had additonal bad sectors...).

3.	Install Xenix.  Modify the hard disk parameters to indicate that
	the number of sectors per track is 26 (not the normal 17 for MFM).

4.	Run the Xenix "media check" just to make sure.

5.	continue with the installation as normal.

			tony....
---
+------------------------------------
|  Tony Field                               ..alberta!calgary!ajfcal!tony
|  Co-Design Information Systems Ltd.
|  Calgary, Alberta, Canada                   voice: (403) 266-3239