Reading ESDI disks

Paul Koning paulkoning at
Thu Feb 11 12:57:19 CST 2021

> On Feb 11, 2021, at 1:46 PM, Chuck Guzis via cctalk <cctalk at> wrote:
> 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.  ...

Then again, ST412 with its buffered seek is vaguely like having a real SEEK command, it's just less elegant because the cylinder number is encoded in base 1.


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