Path: utzoo!mnetor!uunet!van-bc!sl
From: sl@van-bc.UUCP (Stuart Lynne)
Newsgroups: comp.sys.ibm.pc
Subject: Re: RLL on a ST-225
Message-ID: <1683@van-bc.UUCP>
Date: 25 Feb 88 10:12:47 GMT
References: <5037@dhw68k.cts.com> <16800212@clio> <1017@neoucom.UUCP>
Reply-To: sl@van-bc.UUCP (Stuart Lynne)
Organization: Public Access Network, Vancouver, BC.
Lines: 59

In article <1017@neoucom.UUCP> wtm@neoucom.UUCP (Bill Mayhew) writes:
>In article <16800212@clio>, berger@clio.las.uiuc.edu writes:
>
>
>> Most drives intended for MFM work fine with RLL.  Seagate is one of the
>> only manufacturers to advise against it,...
>
>
>The makers of RLL controller cards are also pretty cagey about
>naming drives that will work with their cards too.  There are
>reasons.  The bit rate from a standard MFM encoded drive is 500,000
>bits/sec.  The bit rate for RLL is 750,000 bis/sec.  The reason for
>this is obvious.  Since you can't change the RPM of the drive, and
>the drive has more bits/track, then the data has to move at a
>hgiher bit rate for RLL.  The higher bit rate requires that the
>bandwith of the head amplifiers be sufficent first of all;
>secondly, the media of the disk platter itself has to be able to
>handle the proper bit density.

I'm *probably wrong*, but my understanding of RLL is not that your have *more*
bits being stored on the disk, but simply that your are making more efficent
use of them by encoding your data with smaller/shorter bit patterns. 

You do see an effective data rate that is much higher between the cpu and
disk controller, but *not* between the controller and the drive. 

The reason that the manufacturers rate some drives as RLL capable is
simply that because your are using an encoding scheme with less redundancy
you will have a higher error rate unless you have better media. So you are
paying for (usually) plated media which they test better. 

A similiar phenomenom occurs with AT floppy disks. Diskette manufacturers
issue hideous warnings of impending doom if you dare to use other than
special high density AT style floppy diskettes (which they incidentally
charge a *lot* more for). Most of us just buy the good old cheapies and
simply throw out the 1 or 2 per box which don't work.

Unfortunately with a hard disk you can't just throw it out if it doesn't
work, so probably in most cases if you are buying *new* equipment it's worth
your while to buy RLL rated drives. However if you already have a drive and
happen to get an RLL controller you might as well try it. 

Of course if you end up getting hundreds of bad blocks then trying to map
them out is not a good idea, but if its a reasonably small number you might
consider using it that way.

I'll be trying this out sometime this spring, as soon as the WD-1006 RA2 is
available. And I've got some real oldtimer's that I'm going to try RLL on. 


BTW the transfer rate on standard ST-506 type drives is 5 MBits/second not
.5 MBits/second.

>reasons.  The bit rate from a standard MFM encoded drive is 500,000
>bits/sec.  The bit rate for RLL is 750,000 bis/sec.  The reason for


-- 
{ihnp4!alberta!ubc-vision,uunet}!van-bc!Stuart.Lynne Vancouver,BC,604-937-7532