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From: plim@hpsgpa.HP.COM (Peter Lim)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Re: RLL controllers with MFM drives (exaggerated dangers...)
Message-ID: <340010@hpsgpa.HP.COM>
Date: 27 Oct 89 03:46:23 GMT
References: <1989Oct23.152125.7013@Octopus.COM>
Organization: HP Singapore IC Design Ctr
Lines: 42

Here's my two-cent's worth on RLL.

From what I read from a book titled "????'s IBM-PC Hardware Bible",
can't quite remember the author's name, RLL differs from MFM on two
counts:

[1] The encoding method. RLL re-arrange the data using the 2,7
    encoding method so that you always get a certain number of
    consecutive 1's (I think 5 ??) in a 16 bit word. This arrangement
    does two things: increase the number of raw bits to be written on
    disk for the same amount of data, and reduce the flux density
    for the same amount of raw bits to be written on the disk.

[2] Increase the number of sectors per track from 17 (MFM) to 26;
    which gives about 50% more capacity.

You increase the amount of data to be written to disk and put more 
sectors on one track, but reduce the inter-bit flux density.
According to the author, it works out that for an increase of flux
stress of about 14%, you get 50% more capacity. Reasoning goes that
this 14% extra stress is well within the tolerance of most recent
MFM drives, so they can be used as RLL drives.

My brother has been using Seagate MFM drives for RLL for more than
a year and had no problem at all. I haven't got my own system yet,
but I think I will just go ESDI instead  :-). I hope the above explains
something. Sorry I didn't memorize the numbers and probably phrase my
sentenses badly........ but that's what you get of two-cent's worth
:-).........


Regards,
Peter Lim.
HP Singapore IC Design Center.

      E-mail address:              plim@hpsgwg
      Snail Mail address:          Peter Lim
                                   Hewlett Packard Singapore,
                                   (ICDS, ICS)
                                   1150, Depot Road,
                                   Singapore   0410.
      Telephone:                   (065)-279-2289