Reading ESDI disks

Jon Elson elson at
Fri Feb 12 10:14:31 CST 2021

On 02/12/2021 09:04 AM, Mattis Lind via cctalk wrote:
>   I am a bit confused about the READ GATE. It has to be deasserted
> in the PLO SYNC interval. Why is that? I am clearly missing something.
ESDI has a clock separator built into the drive, with logic 
to decode the data format and convert
to a standard protocol.  Many of the higher-end drives used 
some form of RLL encoding internally.
I'm just guessing here, but I suspect that the sector 
headers were written during a drive format, and then never 
written again, as some recently discussed DEC drives did.  
So, there'd be a synch discontinuity between every sector 
header and the data block.  I'm guessing a drop and then 
re-assertion of read gate would prepare the PLO to resynch 
quickly at the beginning of the data packet.  The controller 
would know when to do this, as it would read every sector 
header looking for the desired sector, and as soon as the 
header data ended, it could wiggle read gate.


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