RLL vs MFM hard drives???
Jon Pardue
Hello again, folks. I've got a reeeeal stupid question for you:
What is the difference between an RLL drive and an MFM drive? A friend
is trying to add a couple of hard drives to his system and he's been told
not to mismatch them with the controllers, but we don't know quite what
that means. Can anybody help?
Thanks!
- Jon
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Jon Pardue "The plural of 'spouse' is 'SPICE'."
par...@rainbow.ecn.purdue.edu - Charlie Gordon
"Old musicians never die, they just go from bar to bar." - Anonymous
Marc Jellinek
>
> Hello again, folks. I've got a reeeeal stupid question for you:
>
> What is the difference between an RLL drive and an MFM drive? A friend
> is trying to add a couple of hard drives to his system and he's been told
> not to mismatch them with the controllers, but we don't know quite what
> that means. Can anybody help?
>
RLL and MFM drives use different, although similar encoding schemes to
store info on the hard drive. I think think what your friend was warned
not to do was hook up an MFM drive to an RLL controller, and visa versa.
You can get an MFM drive to work on an RLL controller...but it is VERY
risky, as RLL drives need to conform to a more demanding spec.
Dave
{>
{> Hello again, folks. I've got a reeeeal stupid question for you:
{>
{> What is the difference between an RLL drive and an MFM drive? A friend
{> is trying to add a couple of hard drives to his system and he's been told
{> not to mismatch them with the controllers, but we don't know quite what
{> that means. Can anybody help?
{>
{> Thanks!
{>
{> - Jon
{> --
MFM is encoded at 5.0 Megabits per second
RLL is encoded at 7.5 Megabits per second.
-David
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Mike Thomas
>{>
>{> What is the difference between an RLL drive and an MFM drive? A friend
>{> is trying to add a couple of hard drives to his system and he's been told
>{> not to mismatch them with the controllers, but we don't know quite what
>{> that means. Can anybody help?
>{>
>
>
>RLL is encoded at 7.5 Megabits per second.
>
MFM stands for 'modified frquency modulation' while RLL is 'run length limited'.
The previous poster was correct they are both encoding techniques. The following
is from Micro House's encyclopaedia of hard drives:
"It is important that you select the correct type of encoding when installing
a hard drive and controller. MFM drives always have 17 sectors and RLL usually have
26 or more. Even though the firmware on an MFM interface card will allow you to
enter in a sector value greater than 17, the electronics will only use 17. A 30
MB RLL drive used on an MFM controller will only format out to 20 MB, since it is
using only 17 sectors per track instead of the optimum 26.
Don't go the other way either! Don't ever use a hard drive designed for MFM
with an RLL controller. The MFM drive was designed to use only 17 sectors per
track and forcing it to use 26 or more could permanently damage it. You may get
30MB out of a 20MB drive, but it won't last too long."
I have heard of people getting away with using MFM drives and RLL controllers, but
I think they're more likely the lucky few. What seems to happen to most is that the
format routines work fine, they load the data and everything seems ok, but after
a few days or weeks all of a sudden they can't access their drive or they experience
data corruption problems.
Basically any good drive 'bible' or 'encyclopaedia' will list the various drive
models and their encoding techniques. These same books will also usually list the
encoding technique employed by the various disk controllers. Just look up your
drive model types and be sure the encoding technique is the same for both disk and
drive. Any local service organization should be able to help too.
Regards, Mike Thomas
Stanley T.H. Chow
>>MFM is encoded at 5.0 Megabits per second
>>
>>RLL is encoded at 7.5 Megabits per second.
>>
>Geez, why even bother to post, if this is all the more info you can provide?
At least, his information is correct! Your information is wrong.
>MFM stands for 'modified frquency modulation' while RLL is 'run length limited'.
>The previous poster was correct they are both encoding techniques. The following
>is from Micro House's encyclopaedia of hard drives:
So far so good. But your source is wrong.
Both RLL and MFM are forms of encoding. Actually, RLL is really short
for RLL 2,7; and MFM is a dengenerate form of RLL (aka RLL 1,3). What
these things do is "encode" information into electrical pulses. The
pulses are then passed via the ST412/506 interface, so that the drive
can turn the pulses into flux transitions. When reading, the processing
is reversed.
Note that the drive only ever sees electrical pulses coming through the
same ST506 interface. There is absolutely no way that a RLL controller
can damage an ST506 drive.
The *only* difference between MFM and RLL is that
RLL has slightly tighter timing margins. In theory, it is possible for
a ST506 drive to meet the MFM margin but not the RLL margin. In that
event, the worst that can happen is lots of retries.
> "It is important that you select the correct type of encoding when installing
>a hard drive and controller. MFM drives always have 17 sectors and RLL usually have
>26 or more. Even though the firmware on an MFM interface card will allow you to
>enter in a sector value greater than 17, the electronics will only use 17. A 30
>MB RLL drive used on an MFM controller will only format out to 20 MB, since it is
>using only 17 sectors per track instead of the optimum 26.
These statements are confused. In particular, the effects of the BIOS
setting are mixed together.
> Don't go the other way either! Don't ever use a hard drive designed for MFM
>with an RLL controller. The MFM drive was designed to use only 17 sectors per
>track and forcing it to use 26 or more could permanently damage it. You may get
>30MB out of a 20MB drive, but it won't last too long."
As I said above, this is plain WRONG.
>I have heard of people getting away with using MFM drives and RLL controllers, but
>I think they're more likely the lucky few. What seems to happen to most is that the
>format routines work fine, they load the data and everything seems ok, but after
>a few days or weeks all of a sudden they can't access their drive or they experience
>data corruption problems.
This is really poor. They hear of people doing it, then they *think* bad
things happen! If you check the people who post their experiences, a lot
people report it works, NO ONE reports that it fails. (A lot of well-meaning
people post INCORRECT warnings similar to yours).
>Basically any good drive 'bible' or 'encyclopaedia' will list the various drive
>models and their encoding techniques. These same books will also usually list the
>encoding technique employed by the various disk controllers. Just look up your
>drive model types and be sure the encoding technique is the same for both disk and
>drive. Any local service organization should be able to help too.
At least this part is mostly good advice.
--
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