SCSI to IDE

[allison]

On 12/04/2010 02:33 PM, Tony Duell wrote:
>> SD cards are attractive in many ways.  For one thing, they can be
>> accessed via an SPI interface which saves some I/O pins on that end.
>> Since I really know nothing about SCSI, I don't know if that is a good
>> approach for a SCSI/SD adapter or not.  I wonder if there are any real
>> time requirements in a SCSI interface that would be impacted by talking
>> to the SD card in a bit serial fashion.
>>      
> As far as I know, there is nothing in the SCSI spec that specifies a
> minimum data rate or a maximum seek time. Soyou could even read 1 byte
> out of an SD card and then send it over the SCSI interfce, and then do
> the net byte, and so on. It would probably be better to buffer a 'sector'
> in RAM on the microcontroller, and then transfer the buffer to the SCSI
> interface.
>    
Correct, same for IDE and CF.  though in those cases you poll or wait
interrupt for ready.  Or in the SCSI case wait for ATN (poll or interrupt).

The oddity is that SCSI treats block IO the same if slow data rate or to
the medium limit. generally slow can be very slow.

Sector buffering is not needed for SCSI, CF, SD or IDE as all have their 
own local buffers
and for the SCSI and IDE those buffers are a minimum of one cylinder in 
size (for the oldest drives)
and for later drive can be many hundreds of KB or even several MB.

However, it can help if the controller is doing a interleaved operations
as block Io overhead is generally lower that byte IO.

So the performance is really define in the bridge device rather than the 
end
storage devices up to the slowest in the pipeline(bottlnecking).

> However, it's entirely possible that some OSes will moan if the device is
> too slow because they will assuem there's a hardware failure. However, I
> would think the time taken to read/write a 'sector' from an SD card would
> nopt be any longer than the seek time of some hard drives. So there
> should be no problem.
>
>    
Possible but not likely as most would assume speeds typical for the time
(slow) and even slow SD cards are faster than that.

Speed in the scsi to anything case is rarely an issue beyond the fact that
slow devices will be a performance bottleneck.  But for systems using
SCSI or SCSI1 we are not talking data rates exceeding a few hundred
Kb/Second.

Where CF and SD cards will excel even with slower data rates they have
no real seek time and for typical disks seek to data rate is usually 
defining
the performance for transfers greater than one cylinder (typically 20k to
100Kbyts depending on vintage).


Allison