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Although the ST-506
drive is no longer the first choice foe
even moderate capacity single-user
systems there are a great many in use and
it still represents a well tried
standard. Indeed , real problem
with the ST-506 is that is interface
with the controller is so well defined
that there is little scope for
improvement. If you where to try to
increase the data transfer rate
say then the result wouldn’t be an ST-506 drive and it wouldn’t work with any
of the many ST-506 disk controllers!
There are two slightly
different types of ST-506 drivers – MFM
and RLL. Indeed these two designation are often used as alternative names for
ST-506 drivers. The difference between
the two types is to do with the
way that
data is coded onto the disk as magnetic
pulses or flux reversals.MFM
(Modified Frequency Modulation) uses an average of 75 flux reversals per bit
whereas RLL (Run Length Limited) coding uses an average of 25 flux reversals.
This means that an RLL drive/controller
combination will store more data in the same space. Strangely enough this increase
in storage capacity isn’t of much interest to and end user (it is to a
disk drive manufacturer however!) The reason being that if I offer you two
drivers that store 100 Mbytes, does it really
matter how this is achieved?
What is really important about RLL drivers it that they offer a potentially faster data
transfer rate to MFM drivers. (There is
a subsidiary issue of reliability its often
said that RLL drivers are less
reliable than MFM drivers because they
store data at a greater recorded-bit density. In practice this difference seems
not to matter as long as the drive is
indeed an RLL certified drive and
not an MFM drive that has been
pressed into service as a cheap way of gaining 50% more storage.)
The reason that RLL drives are faster is that a
cylinder represents are amount of data
that can be read in one
revolution of the drive, i.e the amount
of data that can be read in roughly 16ms
in the case of most PC hard
disks. Thus the larger the capacity of a
cylinder or track the faster the
potential data transfer and RLL
coding crams more data into each track.
The maximum data transfer rates
achievable with MFM and RLL are 0.6 and 0.9 MByte/s respectively.
