Reading ESDI disks

Chuck Guzis cclist at
Thu Feb 11 12:46:56 CST 2021

On 2/11/21 10:22 AM, Jon Elson wrote:

> No.  ESDI is similar to MFM, there is a differential read data pair on
> the 20-pin connector, but the data rate is much higher, 10, 15 or 20
> MBit/sec, and used a different encoding scheme that gave more data
> bits/flux transition.  I think the 34-pin connector also allowed an
> additional head select bit,
> so you could have up to 16 heads.
> ATA has a single 40-pin connector with a 16-bit parallel data bus for
> all command and data transfers.

Brevis esse laboro, obscurus fio.

Let me be more concise.  On an x86 PC, *most* ESDI controllers present
an interface exactly like ATA, right down to the IDENTIFY command.

If you're talking about the naked drive, it isn't ATA, nor is it similar
to MFM.  ESDI drives are "smart" in the sense that they do have a
command set (not ATA, however) and perform tasks such as data separation
and seeking (one issues a SEEK command to the drive, not a STEP IN/OUT).
ST506, in contrast, is a glorified floppy drive with differential data
interface--it has no smarts.  In addition, ESDI also pertains to certain
early optical drives, which ST506 does not.  I believe that there was
also a possibility for a tape interface.


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