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The ESDI
interface, like IDE, is also a
modification of the ST-506 interface but in this case the controller is still a
separate entity. This means that you can buy ISA, MCA and EISA ESDI controller
and so take full advantage of whatever
type of external bus your machine
has. ESDI drives and controller use the same
cabling as ST-506 drivers and controllers, making them physically almost
indistinguishable. Indeed they are also
similar internally and this means that no special software is needed to run ESDI
drives-they are installed and configured
in exactly use same way that ST-506
drivers are. This means that ESDI drivers can offer large storage capacities –
up to 1 GByte – with fast data transfer rates without the fear of incompatibility. The data
transfer rate for an ESDI drive can be
as high as 4MByte/s but 1.25MByte/s is
more typical.
High capacity ESDI
drivers have to use the translation mode
feature as described in the section on IDE drives. This enables them to conform
to the MS-DOS limitation that a drive
must have fewer than 1024 cylinders by pretending to have more heads or more
sectors per track. In this case in translation in performed by the ESDI controller. Because
translation wasn’t necessary in the
early days of ESDI this may make it impossible
to use the larger ESDI drivers
with order controllers.
